- Combining assessments and reassessments
- Is there a definition of a minor mathematical error?
- Requirement for Excellence in "Copper in a Dime"
- Minor errors
- What grades are possible in reassessments?
- Systematic format in recording titration results
- Required calculation in Part B
- Is there a requirement for the mole ratio to be other than 1:1?
- Can Part B Q2 be split into more leading questions?
- To determine the mass of iron in one iron tablet - should this aim be rewritten?
- Typing error on detailed marking sheet
- One minor error in the calculation of the concentration
- Is student 6 an achievement?
- If a student is carrying out a redox titration for 3.1, can this same evidence be used for achievement in 3.2?
- Significant figures
- Significant figures
- Assessment schedule progression
- If a student fails to convert mL to L in a titration is this a major error?
- There appears to be no need for systematic recording of titre values as per AS 2.2. Is there or should there be for M or E?
- Student 4 has no mention of units for mL anywhere
- There appears to be an inconsistency between the two tasks for 3.2.
- Inappropriate selection of titre values
- Major and minor errors
- A query about the "Epsom Salts " assessment
- Can you tell me about the exemplars for Chem 3.2?
- Problem with copper sulfate pentahydrate solution
- Redox internal for a partially sighted person
- There is no mention of acidifying the permanganate solution in "How much iron"
- What if students erase their titration data?
- Minor errors
- Minor errors
- Significant figures
- Significant figures
- Can students use gievn data?
- Can you combine evidence in an initial assessment with a reassessment?
- Residue left after dissolving a ten cent coin
- Issues with making KI solution for bleach titration
- What is my strange precipitate in the determination bleach concentration titration
- Can one do KMnO4 and ferrous ammonium sulfate as 3.2 assessment?
- In the bleach titration is it necessary to use 10 mL of KI?
- Recording data in "a systematic format"
- Fe uptake and Se analysis
- Errors in rounding
Q1. I'd be grateful if you could clarify a point. Can pieces of evidence be amalgamated over an initial assessment and a reassessment in order to assign a final grade? For example, in the first assessment of the redox titration 3.2, one student completed all calculations correctly but did not express the final answer to three significant figures and so missed out on Excellence. In the reassessment, the student followed the correct procedure for completing the calculation, but the final answer was out by a power of ten. However, the answer was expressed to three significant figures. Can this student then be assigned a final grade of Excellence by using evidence from both assessments?
A. Where the standard itself contains two separate criteria (as most of our internals do) it is OK to amalgamate evidence between two different assessments e.g. Meet the requirement for the practical aspect in one task and the theory component in a separate task.
However, it is not appropriate to combine bits and pieces of evidence from different tasks within one particular criteria.
E.g. consider a student who sat 3 different versions of a calculation question. In the first they had no idea of the process but did use 3 sig figs, in the second they used the mole ratio correctly but used C = n x V and in the third they did the process correctly but did not use 3 sig figs.
It would not be appropriate to award an excellence to this student as there can be no confidence that the student is capable of carrying out the entire process² as required by the standard itself. Remember when unsure go back to the title and the criteria described for each level of achievement.
In your specific example used, in neither case has the student ³accurately calculated the composition of the sample and so they cannot be awarded excellence. Back to top.
Q2. The judgement for achieved states that no more than 1 minor mathematical error is acceptable. Is there a definition of a minor mathematical error, or a list of such errors listed somewhere, so that there is consistency in marking. Is getting the formula wrong a minor mathematical error?
A. Unfortunately, no such magic list defining errors exist although the training manuals do give some guidance in this area. Depends what you mean by "getting the formula wrong".
If you mean the formula for water was written as HO, in this context this would be a minor error. If however you mean that they write C = n x V then this is a major error. Similarly incorrect application of mole ratios would be a major error. Minor errors tend to be careless (often related to the maths) whereas major errors involve incorrect processes or methods.
Essentially the question you should ask is "Can they carry out the required process correctly"? A minor error will give the wrong numerical answer but follows a correct process or method. Back to top.
Q3. I am about to do 3.2 and have looked at the exemplars on the net. I am confused by the requirement for Excellence in "Copper in a Dime". The Assessment Schedule states that students must provide evidence of being able to carry out calculations involving a mole ratio that is not 1:1.
The problem is that the reaction does have a mole ratio of 1:1 for Cu2+ : S2O32-. How do I get around this?
A. Two answers here.
- If you have chosen a reaction that does not involve a non 1:1 molar ratio, then an additional theoretical question would need to be offered (either at the same time or in a separate assessment opportunity) to allow students to demonstrate the requirement for excellence.
- Alternatively, in this copper exemplar there are actually two separate equations which individually do meet the requirement, although the overall equation reverts to a 1:1 ratio. So provided the overall ratio was not provided in the task and students were required to do this themselves then they have effectively met the requirement for excellence already. Back to top.
Q4. I have just completed Chem 3.2, all students (7 of them!!!) completed the practical titration section with excellence but 5 of them did not get the calculations - regardless of the number of of times I went over the way to do them. I have had to give them a repeat for the calculation section - a completely new question, 2002 bursary, Pg 23 . One girl has got the calculations correct but instead of having the mole ratio as 5:1 she has 1/5:1, in other words has got the mole ratio back to front but knew what to do and has everything else correct. I think I should give her achieve. Your advice would be very much appreciated.
A. The mole ratio is considered to be a major error and therefore this student has not met the requirements for achievement on either occasion. If you believe that she is capable of carrying out these calculations at some later stage in the year it would be appropriate to assess the calculation portion of the standard again. Back to top.
Q5. I'm just about to reassess those students who didn't manage to reach achievement standard in the practical component of AS C2.2. If they pass, can they be awarded the grade they get (subject to their performance in parts B and C, of course), or are they limited to achievement only?
A. Any grade is possible. Back to top.
Q6. Neither group was happy that there is no requirement for systematic format in recording titration results. Both groups felt this did not follow on from Level 2 and would like it included in Level 3. Please explain the reasoning for this.
A. The recording of results in a retrievable form is really implicit in the requirements of the achievement standard. If an assessor is not able to ascertain where the results have come from then they could impose a 'penalty' for this. The more detail that is in the achievement standard, the more 'picky' it becomes and this means a student who has a reasonable ability could easily be denied. What is systematic format anyway? If this had to be described perhaps many of the level 2 students would not achieve. It is best to interpret the results as retrievable and able to be followed at what is appropriate for Year 13, ie level 8 of the curriculum and not end up making a decision on something that might be quite minor. Back to top.
Q7. Please clarify for me, the required calculation in Part B, the way I read the title and the achievement criteria, it is very clear to me that the calculation (Part B) must be for the titration carried out in Part A. Therefore, we cannot give students an alternative calculation to meet the criteria, despite what it says in the handbook.
A. Yes, the intent is for the student to be able to carry out the calculation appropriate to the data collected. Back to top.
Q8. Is there a requirement for the mole ratio to be other than 1:1, for some reason I thought this was the case, but it is definitely not specified in the Achievement Standard, so is 1:1 OK for all levels (A,M,E)? Can this be clarified in the standard? Especially since for L2 students can't get excellence with a 1:1 mole ratio.
A. There is not really any need for the standard to say what type of mole ratio should be involved. The standards are not designed to be descriptive to that extent. However, as this standard is level 3 it relates to level 8 of the curriculum and therefore reactions that are appropriate to this level. It is important to consider this when choosing an appropriate assessment. At level 8 of the curriculum it would be expected that a redox titration calculation would involve ratios more complex than 1:1. Back to top.
A. No, the questions should not be more leading than given in the template. Again, this reflects the level of the curriculum where students are expected to have a higher level of understanding than is implied for the titration activity as given in the level 2 achievement standard. Back to top.
Q10. The iron titration has an aim stating “To determine the mass of iron in one iron tablet”, but at no stage do student have to carry out this calculation. Should this aim be rewritten? (I did for the purposes of the training days, saved a heap of discussion)
A. Yes – the aim could easily be re-written. The task simply stayed as close as possible to the one which was originally on the web. Back to top.
The generic version does say that on page 29 of the handbook. ie Correct process for determination in gL-1. Back to top.
Q12. The flowchart on p32 says students are allowed one minor error overall. My understanding is that one minor error in the calculation of the concentration in mol L-1 limits the student to achievement. Please tell me this is an error!
A. One minor in determination of mol L-1 does limit student to achievement. If this is correctly determined and there is an error, but correct process, in determining gL-1 merit could be achieved. Back to top.
A. Student 6 does in fact have more errors than initially noted. Not only is this subtraction incorrect but there is also an incorrect transference of values. Achievement would be a fair outcome, not merit as indicated on page 122. Back to top.
A. No, although the same analysis can be chosen to make into an investigation. However as it will be used for a different purpose the actual data obtained form 3.2 cannot be used. Back to top.
Q15. People down here were very happy with the appropriate number of significant figures as per the standards. They are very unhappy with the statement that 3sf is the appropriate number. Can I have a clear explanation of the reasoning behind this decision please to pass onto these teachers? We wasted 40 -50 minutes on this on Friday, I couldn't deflect them and they are not keen to accept any dictate in this regard.
A. 3 sig figs is the usual expected of analysis data although it may be that 4 sig figs could also be the acceptable in some cases. It depends on the raw data that is being used. One would expect a standard solution to be accurate to 3 sig figs and it is only if a mass of less than one gram is used (and a balance which only reads to 0.01 g) than any less would be acceptable. The using of 3 sig figs has been the acceptable standard for a long time - see past Bursary papers for examples. Back to top.
Q16. Here is a statement from one teacher, “Significant figures is a poor way of discriminating chemical knowledge between excellence and merit”. I realise it is not the only thing looked at the excellence level, but sometimes, it all that keeps a student from excellence. I would be keen to hear what you have to say about this statement.
A. For the excellence student it is expected that they understand the accuracy of data and this includes the use of sig figs. Without understanding of this it is quite appropriate to say that the student is not of an excellence level. However, it is important that the data that is provided is of the same accuracy in order for the student to show this understanding. Back to top.
- Achieved - Caculation of titration sample concentration.
- Merit - Calculation of concentration of the original sample and correct procedure for calculation of g/L.
- Excellence - Calculation of correct concentration in mol/L and g/L and 3 sig figs understanding.
Question:- Is the calculation of g/L required for merit in this standard? There are situations where it would be inappropriate to calculate this but does the standard require questions to be limited to those which do involveg/L calculations?
A. Yes, however, a minor arithmetic error allowable for A.
The determination of g/L may not be appropriate in all cases. The 2 activities used were the ones that were on the web - as was required in the preparation of the Manual, and this is what they were written as. Commonly units of g/L would be what is used, and stated, on a commercial product, hence this requirement. However, there will no doubt be some examples where this would not be the best choice in describing the original sample. Where g/L is required is the procedure that needs to be correct for merit, ie recognising the use of molar mass in changing from mol to g. Back to top.
Q18. If a student fails to convert mL to L in a titration is this a major error? If the student fails to do it twice, the errors cancel and the correct answer is obtained. We think it should be a major error. Your thoughts?
A. Agreed that failing to covert volumes twice leads to the correct answer and this is poor work. However, this has been considered as one error thus limiting level to Achievement. Although this may seem generous it is still a method which leads to a correct value and maybe it could be also seen as a ‘lazy' way but one which the student recognises will get the correct value. Back to top.
A. There is no requirement in the standard as such, but you would expect to be able to find the titre values (and final/initial readings) given that this is part of carrying our the titration and getting some results. Back to top.
A. Only for excellence are units mentioned. This then could apply across the whole piece of work not just the final answer. So that if there were no units till the end and a good value was obtained, it could be cut back to merit. Units and significant figures can be treated in a similar way - both expected correct on the final answer and sufficient evidence throughout the whole piece of work to demonstrate understanding of both of these aspects. Back to top.
Q21. There appears to be an inconsistency between the two tasks for 3.2. Does the concentration have to be given in g/L? If so why? The standard only states "accurately calculate the composition of the original sample". Could teachers remove Part B4 and not incur the wrath of the moderators? The group suggest that in the Bleach titration students are being asked to do more - 4 steps compared to 3 (the inconsistency) - to get the original concentration, than in the Iron titration. Could students just be asked to work concentration out in mol/L? this of course relates mainly to Excellence.
A. At this level it is expected that students will be analysing a solution with some relevance. Therefore it is usual to assume that the original substance will have been diluted and that the original concentration therefore needs to be determined and as a commercial product will have the active ingredient quoted in g L-1, it is a reasonable expectation that this s determined. Obviously every analysis will not be identical in what is done during the related calculations just as different analyses will have easier of more difficult end points. It is not unreasonable to allow teachers to use their professionalism to provide a suitable activity for this level. It needs to be appreciated that this is different for the level 2 titration, since we are now working at level 3.
It is not possible to compare each little step but rather to look at providing a suitable activity for a Level 3 standard. As a facilitator we cannot comment on how the moderators will make decisions when moderating an activity or verifying student work. It is important not to get involved in such aspects as moderators cannot be publicly identified according to their contract with NZQA.
It is the role of the moderation process to comment on whether the activity is suitable for the standard and fair for the students. Back to top.
Q22. The explanation of minor errors on page 35 states “inappropriate selection of titre values”. Teachers have questioned whether that is consistent with the statements in the standard with respect to using only specific titre values to determine level of achievement. With student 5, he chooses to uses all of his titre values, so is outside the range for achievement. However if we use your definition of inappropriate selection as a minor error, he is still eligible for achievement, despite being outside the range. Clarification please.
A. The "inappropriate" relates to the student not picking the best set of values to average but whatever is selected still needs to be within the maximum range specified by the standard itself. e.g within 0.8 mL
Consider the set of titres 17.8, 16.9. 16.9 and 17.0. Using all four values to obtain an average would be inappropriate but would still meet the range requirements for achievement. Back to top.
Q23. Just a query about A.S. 2.2 as I found conflicting comments that I made on the marking schedule when we went over it during the TOD. In parts B and C where can the major and minor errors be made i.e for A or M? The simplified marking schedule conflicts with the detailed one.
A. Basically parts B and C represent 2 opportunities for the student to demonstrate that they know how to carry out a titration calculation. As a teacher you must have evidence that they can do this (either in B or C). For Achievement you must be satisfied that they can do this although a minor calculation error is allowed eg incorrect conversion mL to L, or incorrect use of calculator. What is not permitted is that they show lack of understanding of the process by trying to go "concentration = n x v".
For merit the student must be able to show that they can carry out a calculation correctly with no errors.
The confusion in what you have in front of you may be due to the fact that a correction was given to the training manual assessment sheets on pages 63 and 64. ie on page 64 under merit it should say "No errors in the calculation of an unknown solution" while for Achievement it is "No more than one minor error in the calculation". Back to top.
In Part C question 5a and b - calculating an empirical and molecular formula from mass measurements. If a student just gets part a correct surely this is a merit grade as its a two step calculation ( and see B5 above). And if a student gets both parts of C5 correct surely the grade would be excellence. However, the grades are actually achievement and merit? Can you throw any light on this?
A. This is a tricky one but I guess the easiest answer is to say that if the task was being prepared today it would be different. Remember the task was written two years ago and we have moved on a lot.
However, in a partial defence I should say that the thinking wrt to the calculation based on the practical work is that although only two steps are actually involved in B5 it is the culmination of all the questions that went before. i.e if the task had not been broken up into 5 different subquestions it would have been at least a 5 step question overall.
In contrast C5 is definitely a stand alone question which involves 2 steps for part a and on the surface involves a third step in part b - but this could actually be interpreted as just an additional one step process because of the way the question is asked. i.e if the question just had one part stating given the following mass composition and molar mass determine the molecular formuale of caffeine then it could rightly be classified as a merit/excellence question. So how you ask or format the question affects the degree of dificulty.
To be honest however,when we were allocating the level of difficulty at the time (2-3 years ago)I believe we simply applied our common guild of knowledge and expectation that phrased as it is this was a fairly standard question for students to answer which didn't justify the award of excellence. Back to top.
A. The two redox titrations involving iron and bleach respectively have had some very minor changes made to bring them in line with the final registered standards and have now been republished on the TKI NCEA web site. So if you were intending to use either of these exemplars for your 3.2 internal I suggest you re-downloand them. Back to top.
We made up the copper sulphate pentahydrate at 17g/L M= 249.5. This is 25.45% Cu= 4.32g/L of Cu The expected amount from the coin is 4.28g/L One would expect an answer of about 75% We are getting 127% !!! because the calculated amount from the titration is 7.2.
Here are my sample answers:
- average titre= 28.5mL, n(S2O32-)= 0.1x 0.0285= 0.00285
- n(I2)= 0.5x 0.00285
- n(Cu2+)= 2x n(I2)= 0.00285, c(Cu2+)= 0.00285/0.025= 0.114 mol L1-
- m(Cu)= 0.114x63.5= 7.239g 5. %Cu= 7.24x100/5.70= 127% (3 sig figs)
A. When your sodium thiosulfate solution was made up, what molar mass was used in the calculation. If the anhydrous mass (158) was used instead of the correct pentahydrate mass (248) your solution would have only been 0.0637 mol/L rather than the intended 0.1.
Following through with your calculations based on the lower value would give a mass % of about 80% which would be in line with expectations. Therefore, I suggest you check the calculations used in the sodium thiosulfate solution and if possible check the conc of this solution (if any is left) against another standard solution.
If this doesn't turn out to be the problem I can't provide any other possibility unfortunately. If this is the problem then obviously the student work can be simply reassessed based on the actual concentration of thiosulfate used being different to that stated. Back to top.
Q27. Could you clarify a point in the redox internal? I have a student who is partially sighted. To write he uses a powerful magnifyer and still sits about two inches from his page. He normally has a reader/writer in class, mainly for work that is written on the board or to help with class practical activities.
My problem is that a titration is almost impossible. He will struggle to see the marks on a burette and stop at the appropriate colour change. Can we provide him with a set of results and allow him to identify the titres to use and complete the analysis? If we can do this, do we limit him to 'Achieved' in this example or is this just not possible for him? The school exam person says that he should be allowed a chance but the Special Ed persons thinks not.
A. Unfortunately, with standards based assessment this sort of compromise is not possible. i.e. It is not possible to award credit for part of a standard.
In this particular situation , we are assessing the ability to carry out a practical titration and if the students disability does not allow him/her to do this then they cannot receive the credit for something they cannot do.
Perhaps one possibility is to have a reader/writer read the scale of the burette for them but they would still need to actually perform the titration procedure themselves. However, I suspect that the difficulty in seeing the end point colour change and the meniscus reading on the pipette woulkd probably mean that their accuracy would be outside the limits required for achievement anyway. Back to top.
Q28. We are doing "How Much Iron" for Chem 3.2 next week. There is no mention of acidifying the permanganate solution in the resource requirements. Is this correct? Will the acid in the iron (II) solution be sufficient?
A. We actually find it is easier to add the acid to the iron(II) solution as it needs to be acidified anyway to help stop it oxidising. In reality for this task we find that to provide sufficient acid to prevent formation of MnO2 during the titration we essentially have to make up the Fe(II) solution in 1.0 M acid rather than water. The other alternative is to add a line into the instructions to add about 10 mL of acid to the solution before titrating.
I agree that info about the need for acidification should have been included in the task - or in the technician notes for making up solutions. Back to top.
Q29. A student has apparently erased their titration data and re-written their results tidily in a table but only showing final titres. I assume that this does not show systematic format as the results cannot be verified and so NA.
A. Correct - although this seems harsh it is to avoid the possibility of students simply making up data. Back to top.
Q30. A student has written 0.1 molL-1 for the concentration of the standardised solution, in spite of 0.102 being indicated on the bottle (no mention of a concentration value was given in the task to avoid this confusion!). Would this be a minor error only and thus limit to A only?
A. I would agree that this is simply carelessness and should be seen as a minor error. Obviously their overall grade can be improved (subject to the quality of their practical work) if they are are able to successfully carry out the calculation in another assessment opportunity such as a test or exam. Back to top.
A. I agree - too much time and effort. When we supervise we always start our instructions by pointing out the actual concentration written on the board and get them to copy this down on their handout in an appropriate place before they start and this seems to have eliminated this problem. Back to top.
A. Agree Back to top.
A. OK assuming the concentration was actually 0.100 and they have carelessly left it off but if the actual concentration was actually 0.102 as implied above then they don't deserve excellence if they have used 0.1 in their calculation rather than 0.102. Back to top.
Q34. We have a query regarding the Redox Titration 3.2. The standard now indicates that given data can be used for the calculation part of the standard. During the training days there was considerable discussion about this and the outcome seemed to be that students couldn't use given data. Was this a recent alteration and are we correctly interpreting the standard. We're pretty keen on providing data for obvious reasons.
A The expectation is that for calculations students should use their own data - why wouldn't they as after all what is the point of doing the titration if they don't use their values to calculate the unknown. However, if they stuff up the calculation from the practical assessment but get valid titration data in terms of accuracy/consistency, then rather than have to repeat all of the practical work in a full reassessment it would be appropriate to give them a set of data to carry out the calculation on and reassess on this aspect only.
This set of data could be provided as part of a regular exam/test situation and means that at a practical level reassessment becomes much more manageable. Back to top.
Q35. I'd be grateful if you could clarify a point. Can pieces of evidence be amalgamated over an initial assessment and a reassessment in order to assign a final grade. For example, in the first assessment of the redox titration 3.2, one student completed all calculations correctly but did not express the final answer to three significant figures and so missed out on Excellence. In the reassessment, the student followed the correct procedure for completing the calculation but the final answer was out by a power of ten; however, the answer was expressed to three significant figures. Can this student then be assigned a final grade of Excellence by using evidence from both assessments?
A. Where the standard itself contains two separate criteria (as most of our internals do) it is OK to amalgamate evidence between two different assessments e.g. Meet the requirement for the practical aspect in one task and the theory component in a separate task.
However, it is not appropriate to combine bits and pieces of evidence from different tasks within one particular criteria. E.g. consider a student who sat 3 different versions of a calculation question. In the first they had no idea of the process but did use 3 sig figs, in the second they used the mole ratio correctly but used C = n x V and in the third they did the process correctly but did not use 3 sig figs.
It would not be appropriate to award an excellence to this student as there can be no confidence that the student is capable of carrying out the entire process as required by the standard itself. Remember when unsure go back to the title and the criteria described for each level of achievement.
In your specific example used, in neither case has the student accurately calculated the composition of the sample and so they cannot be awarded excellence. Back to top.
Q36. Thanks for your help clarifying our question regarding 3.2 titration assessment (see above as covered on NCEA#4). However, I am still a little bit confused. I believe that NCEA offers us opportunities for reassessment in order for students to be able to correct earlier mistakes and 'prove they can do it'. This student had done everything to gain an excellence except quote the answer to 3 sig figs. This is a minor error which could be corrected in a number of ways eg: through reassessment, by referring back to one of her formatives, or even by discussion with the student at a later date. Once she has proved that she can write answers to 3 sig figs, she has, in my opinion, completed the standard.
One of the things that is giving NCEA a bad name amongst parents and students is a somewhat pedantic adhesion to rigid criteria on the part of teachers. Students feel this is unfair, and it is certainly not in the spirit of the original vision of NCEA, which is to reward students for their successes, rather than penalise them for unnecessary errors. I believe that if NCEA is perceived as unfair teachers will face a massive uphill struggle in the coming years.
A. I agree totally with you about the need to not be too pedantic with assessment but would like to make two points:
- The need to show an awareness of 3 sig figs is only required at Excellence level so as such does not impact at all on 90+% of our students. However, when developing the standard originally the writing team (in consultation with teachers throughout the country) did consider that an appreciation of the importance of sig figs was important for excellence - particularly today when student use of calculators has largely meant the loss of the big picture "does this result seem sensible?"
- One of the important changes associated with NCEA assessment is the concept of holistic rather than atomistic assessment. This means that the student has to be able to demonstrate competence of all of the expected outcomes within the required context, i.e could an assessor feel confident of a students ability to perform an acid-base titration if over three diffeerent assessments they successfully managed to perform only one of each of the required outcomes (accuracy, consistency and calculation) in each of the separate exemplars.- I certainly wouldn't. This does mean that a students overall grade is usually limited by their performance in their "weakest" outcome but we can be more holistic with our judgements in sub outcomes. For reasons of practicality and management/workload issues, the concept of an overall holistic judgement of a single assessment event has been relaxed a little to enable the combination of results from the major outcomes of different assessement exemplars (e.g. carry out titration in one task and carry out a calculation in a separate task).
However. I still don't feel it is appropriate to be able to combine a myriad of sub outcomes from different opportunities for the reason stated above - i.e that as an assessor we can't feel confident that the student can actually carry out the overall outcome of the standard as described in the title (and defined by the learning outcomes). Back to top.
Q37. I'm having a problem with 3.2 in that I have followed the procedure outlined in the online assessment to try and get a 10c piece to dissolve. It dissolves on 30 mL on conc nitric fine and evaporates to leave a black residue without problem. However try as I might, I don't seem to be able to get the residue to completely dissolve in strong sulphuric acid. There is always a fine brown looking sediment left despite the fact that the solution still has a pH of 0-1.
Is this sediment NiO? If it is then it probably won't affect the final results in terms of copper concentration. Have you encountered this before or has any one else any thoughts?
A. Yes we couldn't get it all to disappear and hence just focussed on the copper sulfate solution. It doesn't alter the practical itself as once the copper solution is filtered and made up, the titrations will be consistent. It is also probably more sensible to use a goldilocks pot scrub to clean the surface of the coin initially. Back to top.
Q38. The Bleach titration for is to be the emotional and literal death of me, I fear. Last year I freely admit I was a clown in a hurry and made the KI the wrong strength. This year I resolved to be extra careful and follow the web recipe to the letter. All solutions were made with two people in attendance in case one had a mental aberration!
The KI was made 100g in a litre. The bleach was made 1000 ml in 10 L and 2 lots of 200 mL in 2 L. The S2O32- was made to 0.1 mole per litre. 14 litres of the two main solutions were made in two big containers and shaken until I and they were dizzy! I bought new dijipipettes. I had mostly new but all good condition burettes. I checked TKI site to see that it was 5mL of KI to use and 5mL of H2SO4
I did the sums to prove that 5mL of KI is excess for the OCl- reaction.
Then I did my “correct” versions of the titration. I got concordance at 26.4, 26.45 and 26.5 mL. There was a problem of a lot of solid Iodine precipitated, but with lots of swirling it redissolved in the course of the titration
Three classes of student results later, nearly everyone was above the 26.45 mL titre of mine. My colleagues repeated the titration. They agree with each other at 26.8 and 26.85 mL
With a leaden (Pb) heart I redo my titrations after school. Very carefully as before. Again I get concordance - 27.05, 27.1 and 27.0 mL. On the third one I put in 10 mL of KI.
It got rid of the solid Iodine ppt problem but made no difference to the titre obviously. Grief!!!! I have little enough hair as it is. I am getting less. Can you suggest any place I am going wrong?
A. Firstly, the issue with solid I2 ppt. Your bleach solution is fairly strong (with a titration of over 25 mL required you could easily dilute it by two to save chemicals (and time for students and also mean they could get more than one titre out of each burette refill). A titre value of 12-20 is fine for accuracy etc. The strong solution means that a lot of Iodine is generated and as it has limited solubility it will ppt. Also, the excess KI added is not just there to react with the hypochlorite ion – the unreacted excess reacts with the iodine product to produce the soluble I3- ion which helps stop solid I2 precipitating. Therefore in your case, if you don't reduce the amount of bleach used I would definitely increase the amount of KI added e.g. double it. This would largely eliminate the I2 ppt problem although as you say it does disappear as the titration proceeds anyway.
With respect to the variable titration values it does emphasise the need for the assessor to carry out the titration for each and every assessment session – not just once (whether at the start or at the end of all the sessions). As to the cause of the variation in this particular case which I have also experienced, I am a bit stumped. The results show a steady increase in titre volumes with time which suggest either the concentration of the solution in the burette (thiuosulfate in this case) is reducing with time (which I think would be highly unlikely as it is a very stable primary standard) or that the concentration of the bleach solution is actually increasing with time. The chances of the water evaporating faster from the solution than the active ingredient itself seems equally unlikely. Perhaps one of our readers can offer a plausible suggestion. However, no students should be disadvantaged by whatever is going on, provided as stated earlier you do your own titrations during each and every assessment session. Back to top.
Q39. I have a question. I am doing the Achievement standard 3.2 and I am doing the determination bleach concentration titration. Some, not all, of my students get a precipitation of what I believe is iodine, when they add the sulfuric acid to the diluted bleach and KI mixture.
Can you tell me what I am doing wrong, this time? This is the third time through this Achievement standard and I never had this difficulty before. How can I fix it?
A. Because iodine has a limited solubility in water, some iodine will often precipitate out of solution when it is formed from the reaction of hypochlorite with acidified potassium iodide. Whether or not this happens and to what extent depends entirely on the concentration of the bleach solution used. This can vary significantly between different brands and also between an old opened bottle and a new bottle of bleach.
Therefore I assume that this year you are simply using either a different brand or a “fresher” bottle of bleach. However, it doesn't matter at all, as during the titration the solid iodine will simply re-dissolve as the aqueous iodine is used up by the thiosulfate. However, you may find that the average titre required is a bit higher than you might like so you can dilute your bleach solution to suit. Back to top.
A. No problem – this is actually one of the examples shown on TKI as 3.2 _E v3 Back to top.
A. The KI serves two purposes – initially to be quantitatively oxidized to I2 and the quantity specified is significantly more than should be required for this to occur under the stated conditions. However, its presence in excess also allows for the formation of the I3- ion which is significantly more soluble in water than the original non-polar I2 molecules are. Without this significant excess of iodide, the iodine produced tends to form a large amount of iodine solid which is unpleasant and also may escape from the flask via sublimation. Hence my advice is that although you could save on costs a little by reducing the amount of KI added, I don't personally think its worth considering. Back to top.
Q42. The mark schedule for "Iron" says that titration data should be recorded "in a systematic format". A number of our students have recorded titre values only - with no "start/finish" readings. Is this acceptable or not, in your opinion, please?
A. A very grey area as the systematic format has never been formally defined. Normal practice would be to show start and finish values as this allows for some sort of verification of the values obtained (particularly for external moderation requirements). However, it would seem very harsh to "fail" a student simply because of this omission and hopefully common sense would prevail. It shouldn't be a moderation issue in future as the reference to systematic format has been dropped from the revised standard for use next year. However I would still be encouraging students to use this format not only as good practice but because I have used the raw data in the past to identify students who have not read a burette correctly e.g. 39.8 as 40.2 mL Sorry I can't be more definite. Back to top.
Q43. Just have a few Y12 students doing some work on Fe uptake by plants. Have you done any colorimetric work with this down the the appropriate levels? Any knowledge on Se analysis for the same? Just thought I would check before I range further afield. I have a few kids doing Chem 3.1 and some doing Science 3.1 Most are doing silver CREST so have consultants.
A. We have done analysis of iron content in meats and vegetables using the spectrophotometer and the only issue has been stability of the FeSCN+ complex with time. Iron level in some cases have been quite low but detectable. The only method for detecting Selenium listed in Vogels is a gravimetric method so this wouldn't be suitable at the low level expected in plants. I imagine the only option would be something commercial like AA absorption - possibly they could approach the university people. Back to top.
A. I think the rounding of figures, as you have indicated, is not a minor error and the student should not be penalized. However, it is annoying to see this happening and it can result in a different answer. But the chemistry is correct, and the use to 3 sf (and units) is not incorrect, so should not be penalised. Teaching programmes should address this issue. Back to top.