- Grades for reassessment
- Reassessment of practical component only
- Average titres outside limits for achievement
- Error in changing volume from mL to L
- Wording of 2.2
- NZQA exemplars
- NZQA exemplars
- Should we give different students different values for the titre?
- Should we give different students different questions for Part C?
- How long can we leave between the two assessment stages?
- Who can supervise the acid-base titration?
- Silly miskates in Part C
- Common experimental problems in 2.2
- Rounding three concordant results before calculating concentration
- Do we need to give a range of volumes when doing the 1:2 mole problem?
- Clarity of the marking schedule
- Marking 2.2
- Reading burettes and titration accuracy
Q1. 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.
Q2. Two students have achieved excellence in both the theory parts of the assessment, and have also achieved excellence for concordance in the practical, but their mean titres are outside the 1 mL accuracy limit for achievement (one of them is 1.1 mL out, and the other is 1.5 mL out). Can I just get these students to do another titration next week, or should I make them do the whole assessment again next term?
A. Its perfectly acceptable to offer the reassessment as a practical only component as you already have sufficient evidence for the theory component. What happens more frequently is that students perform the titrations OK but stuff up the calculation. In this case there is little work required in offering another assessment opportunity for the theory component e.g. by including it as a component of a school exam.
Re: the accuracy issue. I would strongly recommend that you check the calibration of the equipment used (particularly the pipette) - especially if it is B grade as most school equipment is. Before we carried out our assessment we checked close to 50 pipettes by weighing the 25 mL delivered. We found 3 to be not delivering the expected volume within stated accuracy so we removed them from the assessment. We also numbered all the gear so that if an issue arose we could refer to our calibration list or go back to the equipment and identify a possible problem beyond the control of the student (such as a student in the prior assessment breaking the pipette tip). Back to top.
Q3. Another student has also achieved excellence in the two theory parts. However, he too has failed (I think) the achievement limit. My mean titre was 16.20 mL, and his four values were; 17.0, 17.3, 17.2, and 17.3 mL. He averaged the last three values to get a mean titre of 17.27 mL: just outside the 1 mL range. If he'd used all four of his titres his average would have been 17.2 mL, which is just on the limit for achievement. What should I do? Award him an achievement grade now, or award him non-achievement now? (If you recommend the latter, then he falls in with the first two students, and so should be treated the same as them.)
A. In this case you as the assessor can use all 4 results (as they are within the required concordancy for Ach) and obtain a result which is just within the Achievement requirement. Therefore he is entitled to receive Achievement i.e. his practical ability is not penalised by his choice of values to average.
However if a student is out by 1 mL in an acid-base titration you would have to suspect a large systematic error somewhere e.g inaccurate pipette or as his values are consistently too high perhaps he is blowing out the residual volume from the pipette into the flask each time. Alternatively his interpretation of the meniscus reading could be way off. (note that this wouldn't matter with burette readings as the error would self cancel). To check students ability to pipette accurately and consistently we get them to pipette successive samples of water into a beaker and record the mass each time. They can easily check their own consistency and we provide the expected mass for them to check their accuracy against.
Just a further comment - I note that all readings are only given to 0.1 mL. We expect all our students to read burettes to at least .05 mL and most read to within .02 mL. This extra precision can often make a difference in their final averages and also impresses on them the need to be careful and precise with each titration. Back to top.
A. Yes conversions mL to L is a minor error. Often when they fail to do it twice they actually get the correct answer at the end. Back to top.
Q5. Just one thing when the standards do come up for review, can the wording in 2.2 about range and within the expected outcome be separated out please to clarify it. People are trying so hard to confuse the issue.
A. Totally agree!!!! Back to top.
A. Because the practical component is largely assessed by the quality of their final concentration there is no real need to change instructions except to meet your own requirements e.g availability of different size pipettes etc. Back to top.
A. You can use as is if you feel comfortable that your students won't have accessed the material from the web themselves and learnt the answers by rote.
Personallly we prefer to keep the intent of the question and just tweak the numbers. Very quick to do. Back to top.
If we're doing AS C2.2 in batches (25 students in a batch, all on the same day, but not at the same time), should we give them different correct values (maybe changing both the titres and the stated concentration value for the standard reagent)?
A. We vary the concentration between classes regardless of when they do the task.
You may even want to consider having say 3 or 4 different concs in containers labeled say A, B, C and D so that adjacent students do not have the same concentrations. Could still use the same 4 solutions for other classes but change labels to W,X,Y,Z etc. We also do this for the qualitative analysis. Back to top.
A. This is preferrred where possible to avoid collusion between periods. In SFC it was not such a big an issue as students did not tend to help others as it meant their own grade suffered. Back to top.
A. No set time - could be a week or a month but remember the intent is to assess at an appropriate time when the students are ready. As the two parts cover the same concepts if you had too big a time delay you may have to re-teach/reinforce the concepts to be fair to the students. Back to top.
Q11. Is it a requirement for Achievement standard 2.2 (acid - base titration) that the supervisor be a chemistry teacher or a science teacher? I cant find this stated anywhere but presumably, since the supervisor is asked to carry out the titration before the students, they would have to be at least a science teacher with some chemistry knowledge.
A. The supervisor could be a non science person provided there was a system by which the concentration of any unknown used could be accurately determined for each session. This could involve a lab technician doing the titration to determine the concentration or a chemistry teacher spending 5 min prior to or just after each session offered. Back to top.
Q12. Two students have got all of Part A and Part B to excellence level. Unfortunately they have done silly things in Part C (one has rounded the moles of the standard solution so that her answer is 0.0800 molL-1 instead of 0.0842molL-1 and the other didn't copy the 10-3 from her calculator so her answer is 84.2 instead of 0.0842!) However, both have used the ratio of 2 to 1 correctly even though the values they were using were incorrect. Does this mean that they have got the excellence standard? Or does the statement ' The student must show they can correctly use the 1:2 mole ratio in a calculation' mean they have to get the whole calculation in Part C correct?
A. Looks like a case for professional judgement to me. The evidence to meet the requirements of the standard is all there - just not in one entire piece. .i.e no errors in Part A and B but no opportunity to meet 1:2 requirement presumably, whereas Part C demonstrates the ability to calculate a 1:2 ratio correctly but includes careless "minor" mathematical errors which were not present in Parts A and B. Normally, for investigations and even calculations we would expect to see the correct process carried out in its entirety (ie in context) but as a teacher if I was convinced from everyday classroom evidence that the student was consistently able to perform these calculations I would probably accept the evidence.
However, I'm not sure how an external moderation process would see it. Possibly it would be safer to just give them another calculation exercise under assessment conditions - if they are genuinely excellent students they should be able to do this in 5 - 10 minutes max!
Another possibility for future reference is to use an actual titration that has a 1:2 ratio in it so that there are 2 separate but complete opportunities to meet the requirements. Back to top.
Q13. Hi, as an experienced teacher I would like to ask you what experimental problems are common when doing 2.2. ie) they misread the burette or poor pipetting. I noticed you had them use filter paper to read the burette.
A. Common student problems include:
- Not noticing an air bubble in burette tip.
- Reading burette with eye level above or below solution level.
- (Note that reading meniscus to wrong position is self correcting for burettes but not for the pipette volume.)
- Leaving funnel in top of burette and/or not waiting long enough for solution to drain down to new level after refilling
- Reading burette scale “backwards” i.e. reading 16.8 as 17.2
- Not reading to .02 (or at least .05 mL)
- Adding excess indicator.
- Blowing out pipette into the flask instead of dipping tip below solution level or touching side of flask and discarding residue.
- Not pre-rinsing burettes and pipettes with the solutions being used to fill them.
- No rinsing flasks with water.
- Mixing up which solution goes where.
- Sucking up solution/water into safety pipette filler which later drips back down.
- Common teacher problems include:
- Not ensuring bulk solution is thoroughly mixed before distributing individual samples to students.
- Leaving bulk solutions such as NaOH open to reaction with carbon dioxide from the air.
- Not carrying out the titrations themselves during the time span of the assessment itself.
- Not recording in some way which specific gear (burette and pipette) was used by any individual problem to allow cross checking later for problem such as chipped tip, abnormal volume etc. (When there is any doubt pipette accuracy can be checked by weighing their aliquot of water and using known density value to determine volume). Back to top.
“A standard solution of 0.180 mol L -1 hydrochloric acid was titrated against 25.0 mL samples of a solution of sodium carbonate. The following volumes of hydrochloric acid solution were used in successive titrations – 24.50 mL, 23.25 mL, 23.35 mL and 23.28 mL.
Use this information to determine the concentration of the sodium carbonate solution.”
The answer I was expecting was: 0839 mol L -1 This is obtained by carrying the answer in the calculator through all steps (i.e. holding the max number of significant figures then rounding to 3 at end). Obviously the non concordant result was not included in the calculations.
However a number of top students put 0.838 mol L -1 and this came from rounding the 3 concordant results to 4 sf (which makes sense really since that is the accuracy coming off the burette) and then carrying all the other calculation sig figs in their calculator and rounding to 3 at the end.
Some other students have rounded to 3sf at each stage and therefore have ended up with 0.840 mol L -1
Can I accept the latter 2 answers considering that getting sfs right is a requirement for excellence and this question would swing it either way for some students?
A. I personally would accept either answer. Back to top.
Q15. Thanks again for the help on 2.3. We are now doing 2.2. We usually do the ethanoic acid/NaOH titration and then give the students a 1:2 mole problem to do. If the students show in the first part that they can choose concordant results in their calculation is it essential that we have to give them a range of volumes to select from when doing the 1:2 mole problem? We always find that there will be at least one student that uses all the volumes to work out the average volume but solves the problem correctly, in theory getting excellence.
A. I agree it can be a bit of a problem matching the evidence from the two different tasks. Although we still require them to work out the average of the concordant titre and then follow up with some extra questioning if there is a mismatch between the two tasks, my personal feeling would be that it should be acceptable to just give the average titre value in the 1:2 problem. The important thing is that you are confident as the assessor that they have mastered that particular skill. Back to top.
Q16. I'm just wondering if you could give me some advice about some problems we had with the clarity of the marking schedule for the Y12 titration. The students did a practical titration which they calculated the concentration for, then they were given some data and calculated a second concentration.
1. If a student switched the volumes (used the ‘known' volume for the ‘unknown' solution, and vice versa) and did this both times (in calculating the concentration from their own titration, and also when given data to calculate a separate concentration)…would you give them Not Achieved? i.e. regard it as a major error and hence they failed to calculate concentration?
2. If a student used the opposite ratio (i.e. multiplied by 3 instead of dividing by 3) when calculating the moles of ‘unknown' BOTH times would you give them Not Achieved? i.e. regard it as a major error and hence they failed to calculate concentration?
3. Can we get students to re-sit just the practical titration part (if they got the calculation part correct)?
A. Simple answer for Q1 and 2 is yes – these are both major errors resulting in not achieved grades.
If a student has successfully done the calculations but the accuracy of the titration procedure was inadequate they can repeat just the titration component – however, because it should be seen as a holistic judgement, I would still expect the student to redo the calculation for the repeated titration. Back to top.
Q17. I have a question about marking AS 2.2 (volumetric analysis).. I have two students who have not recorded their initial volumes for their titres. They have just recorded their final titre values (I presume their initial volumes were 0.00mls). In both cases the students would have got excellence and this is their second assessment opportunity. (Both also made the same mistake in their 1 st assessment but had other errors which marked them down. ) Do I mark them down to “achieved” for this error?
A. Strictly speaking they should be limited to Achievement as this Ach Std specifies the requirement to record results in a systematic format but this is 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. Note that this requirement has been dropped from the Level 3 Redox Titration in next year's revised version so it won't be a moderation issue in future at Level 3. 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
However, the current version of Level 2 still has this requirement at merit and excellence levels so I suspect moderators would probably limit the grade to an Achievement.
One possibility could be to gather further evidence without actually repeating the practical titration e.g. give a short written task with three or four diagrams of burettes before and after and ask them to record the data in a systematic format. This would check that they can interpret a scale correctly and require them to record a before and after value to obtain each titre as obviously you could use non zero starting points in each case. Note that students should be encouraged to use non- zero starting points for all titrations anyway.
Perhaps someone out there who has had a case like this moderated recently could advise me what decision was made and I could let everyone know the correct interpretation.
Sorry I can't be more definite one way or the other. This is a typical frustrating example where NZQA's system should allow a definitive answer to be sought from the National Moderator and published for everyone to learn from, rather than each individual school having to find out erratically by trial and error through the moderation system. Back to top.
Q18. AS 2.2. Just a question to which I thought I knew the answer, but the more I've looked into it, the more unsure I've become. My understanding about reading burettes and titration accuracy follows these lines:
Standard burette with 0.1 mL divisions, each reading +/- 0.05 mL (i.e. half the smallest division). Since two readings are required to obtain a titre then each titre has +/- 0.1 mL error (i.e. add the errors).
However, in discussion with other chemists I have heard the suggestion of being able to read a burette to +/- 0.02 mL. If we continue with my original idea, then my understanding is that titres should only contain one decimal place, particularly when an average titre is obtained since they are only accurate to +/- 0.1 mL. Similarly, when completing a titration, titres should be done until two or three titres are within a range of 0.1 mL. Are these ideas still accepted or have I been in error all these years?
A. In answering your question it is worth stopping to think about precision and accuracy. Precision relates to being able read a number off a piece of equipment (it depends on scale divisions and so on). Accuracy relates to how close that number is to the actual value. Accuracy clearly depends to some degree on scale divisions etc, but it is in the way implied by the question "is the 5 mL mark actually at the 5 mL volume?". This will depend on the quality of the burette. Clearly the precision of a reading is irrelevant if the burette quality is so low that the scale is essentially meaningless. Equally there must be scale divisions that are sufficiently small to allow the precision of a reading to make the most of the accuracy of the equipment. In terms of error in a titre, it is accuracy that is most important (provided that the scale divisions that are provided are appropriate).
I may be corrected by my analytical colleagues, but my summary regarding your burette is as follows. Yes, it is possible to read a standard burette to +/- 0.02 mL (which is the precision of the measurement), but the error on the titre volume will be +/- 0.05 - 0.1 mL, because of variations in glassware, placement of the scale divisions and so forth. In principle, all volumetric glassware should come with some sort of certificate of error, but in the absence of that we often use the rule-of-thumb that the error is equal to half the scale division. This essentially means that we are assuming that the manufacturer has chosen an appropriate scale for their quality of equipment. Back to top.