An IB Chemistry question I cannot answer
Sunday 29 May 2016
There has been considerable controversy in the UK recently about the SAT tests in English given to all 10-11 year olds. This was compounded when the tests were leaked before they were due to be taken and many parents withdrew their children from school on the day of the tests in protest. The tests are new and were testing English grammar including the correct use and identification of the present tense, the subjunctive and relative clauses. One very experienced primary school headteacher, Amanda Hulme, said the tests would have stumped Jane Austen. She went on to say, “I have a degree in English language and there are a number of questions that I couldn’t answer.
This year’s IB Diploma Chemistry examination is not an SAT test for eleven year olds but it is also a new type of exam as it is the first to test the new curriculum, which has been taught since 2014. Generally I think the exam was quite fair and it appears to have been well received by students and teachers. However, I find myself in the same position as the primary school headteacher over one of the questions. I’ve got a Ph.D. in chemistry and have taught the IB for many years, written and marked the exams, developed new curricula and written textbooks etc. but I cannot answer one of the questions. The question was Part (c) of Question 2 on Paper 3.
Question 2 concerns a student who wishes to determine the concentration of a solution of sodium hydroxide, which he/she prepared by dissolving 4.00 g of sodium hydroxide pellets in water and making the total volume up to 1.00 dm3. 20.0 cm3 of this solution is then titrated with 0.100 mol dm-3 hydrochloric acid solution using bromothymol blue as the indicator. Part (c) asks, ”Suggest why, despite preparing the solution and performing the titrations very carefully, widely different results were obtained.” (As an aside this is rather a strange question as in part (b) (ii) it states that the student added the acid too quickly, now we are being told that the titration was performed very carefully.)
This experiment is a classic strong acid-strong base titration. Usual sources of error are not thoroughly mixing the prepared solution in the volumetric flask so that each time 20.0 cm3 samples are taken the concentration will differ but this cannot be the reason because the question states that the solution was prepared very carefully. Another problem is that pellets of sodium hydroxide are deliquescent so should not be used to make a primary standard, i.e. the concentration of the sodium hydroxide solution made cannot be guaranteed to be exactly 0.100 mol dm-3. However this is unlikely to give wildly differing results. This point is irrelevant anyway as the aim of the titration is to find the concentration of the sodium hydroxide solution so it is the 0.100 mol dm-3 hydrochloric acid solution that is being used as the standard solution not the sodium hydroxide solution, which effectively is of unknown concentration. Of course student incompetence when doing the experiment, such as not rinsing out a wet burette with the hydrochloric acid solution beforehand etc., might explain poor results but we are told that the titration was performed very carefully so this cannot be the answer. Another source of a systematic error would be if the hydrochloric acid solution (perhaps prepared by a technician) was not exactly 0.100 mol dm-3. If this was the case the results would not be wildly differing, as they should still be precise even though they would be inaccurate and in any case we have been told in the question that the solutions were prepared very carefully so again this cannot be the answer. If the student was colour blind (I am!) they might not see the end point clearly but this is not the fault of the experimental method. The answer is either blindingly obvious and I am just being thick or else some key piece of information seems to be missing in order to answer this question. Of course it may also be that whoever set the question is clear what answer he or she wants but they have not framed the question correctly. Can anyone help?