2.3 Questions and Answers - Chemteach - University of Canterbury - New Zealand

2.3 Questions and Answers Archive

  1. Application of the relationships n=m/M and c=n/V
  2. Distinction between a process error and a mathematical error
  3. Use of significant figures
  4. Determining the requirements of a judgement/sufficiency statement
  5. Significant figures sufficiency

Q1. We have a question regarding AS 2.3.  The achievement standard states for merit, students are required to solve problems that require application of the relationships n=m/M and c=n/V .

Does this mean that we need to be specific on the answer schedule and only award Merit if the student has successfully completed questions demonstrating both these concepts?

At present we are awarding students Merit even though they haven't answered a c=n/V type question as long as the total number of correctly answered Merit questions meets the requirement according to the justification statement. (This appears to be what the exemplar on the NCEA website has done)

If what we are doing is correct then do we even have to include c=n/V type questions in our assessment for AS 2.3 as this skill is inherently necessary for achieving AS 2.2?

A. A quick analysis of Chem 2.3 E v1 on tki suggests that it is indeed possible to get Merit without answering any questions involving concentration at all i.e. Q4, 6a, 6b, 7, 8, 9, 10 and 11 (plus Q1 for an A). The Merit requirement is 6 M plus a further 3 at Ach or higher. However, I suspect the chances of a particular Merit student getting only this combination correct and not also getting any of the 4 other questions which involve concentration correct is very slim. However, your point is valid and probably the exemplar needs a little bit of “tweaking”.

But when in doubt we always have to return to the achievement standard itself rather than associated exemplars. In this case: achievement states the simple quantitative problems may involve …while merit requires application of the relationships …. This means ability to use both relationships must be evident for merit." So any activity that does not include the c=n/V problems would not meet the requirements of the standard as being a suitable activity.

Therefore, in terms of the wording of the standard, a student who is awarded merit is expected to have at least one problem of this kind correct and I assume moderators would interpret it this way. Back to top.

Q2. With the revised version of 2.3 am I correct in thinking that there is no distinction between a process (logic) error and a mathematical (calculator) error i.e that each error effectively takes an individual grade for a question down by one grade.

A. Yes that is correct. As an activity would be assumed to involve multiple opportunities to provide evidence this would not be expected to be an issue. Multiple opportunities would mean that a careless mathematical error would not prevent a student from, overall, attaining a particular level of achievement since a majority of the examples relevant to that level of achievement would be need to be correct. One incorrect step, eg due to a misuse of a calculator, would not influence the overall level of achievement.

The student who has made an error, due to a misunderstanding of the logic (ie chemistry) may be advantaged, but only if they still understand the logic, or chemistry , of the majority of examples offered. Repeated arithmetical errors may prevent achievement. Back to top.

Q3. Use of significant figures. Just some clarification on our assessment schedule, we have 8 opportunities for 3 significant figures in the final answers. How many final answers do you think the students need to give in 3 sig fig for Excellence?

Secondly, it says in the standard that answers must be given to appropriate significant figures. When is it appropriate to use 3 significant figures?

A. The number of sig figs that should be used in any calculation is generally the number of sig figs in the least accurate piece of data. That is why in externals now, virtually all data such as molar masses and masses etc are given to 3 sig figs. If a +/-0.01 g balance is used in an expt, or a titre is read to 2 dp the accuracy is generally to 4 sig figs but the final result may still be limited to only 3 sig figs if another bit of required data such as a concentration or molar mass is only given to 3 sig figs.

However, if they used 4 sig figs in such circumstances it wouldn't be a big deal.

With respect to how much evidence is expected it should be a holistic judgement rather than a mechanical 5 of responses. i.e. are you sure that they understand the concept of sig figs and do they correctly apply it most of the time. Its not meant to be something that is a picky thing to pull students up on but was originally put in to penalize students who either

 -put all the figures off the calculator in

- just used 2 dp regardless of the size of the quantity

- rounded off too much so that most of the inherent accuracy was lost.

If occasionally they use 2 sig figs or 4 sig figs but most of the time use 3 sig figs this is fine. Back to top.

Q4. We are doing 2.3 and I have put an assessment together based on the exemplars that are on the net.  the sufficiency statement is the same as on the net.  For achievement - 7 answers at achievement level etc.

My problem is that I have a student who has only 6 achievement ticks but has answered 2 excellent questions correctly.  The way it reads is that this student won't achieve which I find absurd.  I want to amend the sufficiency statement in line with the way externals seem to be marked now and have a certain number of opportunities answered etc.  I am wondering how many I would need for achievement etc.  At present it is 7 answers at achievement for achievement; for merit 6 answers at merit level + 3 other answers at achievement and for excellence 3 answers at excellence + 4 at merit and 3 others at achievement. 

I am thinking     

Achievement - 8 opportunities answered at achievement level or higher.

Merit - 9 opportunities answered with 6 at merit level.

Excellence - 10 opportunities answered 3 answers at excellence level and 4 other answers at merit level.

One clarification – did the student below only answer 6 items correctly (of which two happened to be at excellence level) or did they answer 6 questions at Ach level only plus two others at Ex level?

No, he answered 10 correctly  6 ticks at achievement level and 4 ticks were in the merit and excellence boxes for 2 of the questions. 

A. When determining the requirements of a judgement/sufficiency statement you could use the following guidelines as a starting point (adjustments may be made to allow for a set of questions that are particularly easy or hard).

Firstly at each level the majority of evidence available must support the decision. i.e at each level, at least half of the opportunities available must be met.

Secondly, to ensure sufficient breadth of knowledge/understanding an increasing proportion of all outcomes should be met for the higher levels. As a rough guide about 65-70% of all opportunities should be met for Merit and about 80% of all opportunities should be met for Excellence. At Excellence the split between more Ach or more Merits is made holistically depending on the nature of the questions.

Therefore for your example which has a total of 12 outcomes – which incorporate 12 A, 9 M and 5 E opportunities I would suggest that the judgement statement could be:

Achievement – at least 7A (or higher) outcomes if the questions are relatively easy –if difficult examples, 6 A or higher may suffice.

Merit – 5 outcomes at M level (or higher) plus a further 3A opportunities (ie obtained from different questions to the other 5)

Excellence – 3 E, plus 4 M plus a further 3 A opportunities (which means at least 10 questions out of 12 are either correct or have partial responses).

This would mean that your student with 6A plus two further examples at E (obtained from different questions to the 6A opportunities) would meet the requirements of Achievement.

There does seem to be some confusion out there about the notion of “double counting” of say a merit result as both an A and a M. Each individual outcome can only contribute once to the overall judgement requirement, but an M for example can be counted as either an A or an M depending on what is required to meet sufficiency.

So in the above example if a student only got 5M and 2A out of the 12 opportunities they will be limited to Ach only as you can't use one of the M outcomes as both an A and an M. Conversely, a student who got 6M and 2 A would meet the requirements of Merit because the 6 th M can be counted as the required 3 rd A.

Hope this helps. Back to top.

Q5. Last year the moderator told us that two sig fig errors out of eight would limit the student to merit even though they got every question correct in paper (just not to right number of sig figs). The moderator said all questions had to be correct. I read the comments about the sig figs on this site which suggests otherwise. Now i'm just confused to whether i ignore the moderator or be more realistic to what I think should be allowed. Can you help please clear this up for me?

A. I would not expect any errors with significant figures in answers to calculations. Errors suggest the student doesn't really understand correct use of significant figures. 

However, a volume that is stated as from a pipette has this accuracy built into it and I would be happy to read it as such. If a volume, say 20 mL is recorded as a pipetted volume, I would be happy to interpret this as 20.00 mL even if this is not stated.  

Often an assessor only looks at the answers leading to excellence opportunities for correct use of significant figures but the requirement for correct usage applies to the whole activity.

There will soon be new activities on TKI for version 2 of AS 90763. You will notice that correct significant figure usage is required for all answers in these activities. I will publicise this when it comes on line. Back to top.
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