11.1

Question 1

11.1 pH - prob• .lem solvingTo use an investigative approach to determine the identity of a number of unknown solutions, knowing that they are solutions of acids and alkalis

Answer 1

You will be given a number of colourless solutions of acids and alkalis. Your task is to plan and implement a practical procedure to work out the identity of each solution.

The following information may or may not be useful to you:

Ethanoic acid has Ka = 1.8 × 10–5 mol dm3
Methanoic acid has Ka = 1.8 × 10–4 mol dm3

Ammonia reacts with water to form hydroxide ions as shown in the following equation:

NH3 + H2O ⇌ NH4+ + OH–

At any one time, only about 1% of the ammonia molecules have reacted to form OH ions.

Question 2

Solutions A–G in no particular order

• ammonia, NH3(aq), 0.1 mol dm–3
• ethanoic acid, CH3COOH(aq), 0.1 mol dm–3
• ethanoic acid / sodium ethanoate, CH3COOH (aq) / CH3COONa(aq), 0.1 mol dm–3 / 0.1 mol dm–3 (1 : 1)
• hydrochloric acid, HCl(aq), 0.1 mol dm–3
• methanoic acid, HCOOH(aq), 0.1 mol dm–3
• sodium hydroxide, NaOH(aq), 0.1 mol dm–3
• sulfuric acid, H2SO4(aq), 0.1 mol dm–3 All solution should be treated as IRRITANT

Answer 2

All solution should be treated as IRRITANT

Question 3

Aqueous hydrochloric acid, HCl(aq), 0.5 mol dm3

Answer 3

No hazard classification at this concentration

Question 4

Aqueous sodium hydroxide, NaOH(aq), 0.5 mol dm3

Answer 4

DANGER
Causes severe skin burns and eye damage
• Wear goggles and chemical-resistant gloves when handling 0.5 mol dm–3 sodium hydroxide.

Question 5

Phenolphthalein indicator

Answer 5

Check the supplier label – hazard depends on concentration and solvent.

Question 6

Analysis of your results

Answer 6

1. Complete a list of proposed identities for the seven solutions. Have the list checked by your teacher. It is helpful to write down all your thought processes in working out which solution is which – this could form the basis for a useful discussion with your teacher and fellow learners.

Question 7

2. Compare the calculated values to your measured values. Are there any differences?

Answer 7

2. As noted, learners may find that their measured values differ from their calculated values.

Question 8

3. What can you conclude about the accuracy of your measurements?

Answer 8

3. Any difference between measured and calculated values shows that measurements may not always be accurate. Learners should consider various reasons for this, and may compare different sets of results. For example, if there are differences between groups who used the same apparatus (assuming their solutions were taken from the same batch), there will have been errors in the taking of the measurements. If values from different groups using the same apparatus are comparable, then the apparatus is not reliable. More able learners may consider errors that may have occurred in making up the solutions.

Question 9

4. Why do you think you were told to collect three pieces of evidence for each solution? Would you have been able to identify all the solutions with just one piece of evidence?

Answer 9

4. Learners should recognise that having multiple pieces of evidence makes it easier to confirm the identities of the solutions.

Question 10

5. How could the experimental procedure/apparatus used be changed to support the collection of more reliable evidence? What are the benefits of performing an experiment like this with very basic apparatus as you have done in this experiment?

Answer 10

5. Experiments involving acids and bases are typically carried out using accurate measuring apparatus such as burettes and volumetric pipettes. Such apparatus ensures consistency and supports the reproducibility of results. However, the use of such apparatus requires careful and precise working and can be time consuming. In this experiment, rudimentary apparatus has been used to speed up the process of data collection in a safe manner. This allows learners to collect a body of evidence in a short time period upon which they can develop their conclusions.

Question 11

6. How effective are acidic and basic buffers at resisting changes in pH when small amounts of acid and base are added, compared with solutions of the weak acid or base on their own?

Answer 11

6. The pH of buffer solutions does change when acid or base is added, but the change is more gradual than when acid or base is added to a weak acid or base. Learners should appreciate that buffers don’t maintain a constant pH when small amounts of acid or base are added (this is a common misconception).

Question 12

note

Answer 12

Learners should collect as much evidence as possible to support their conclusions regarding the identities of the solutions. This will include pH measurements, the effect on pH/indicator colour of adding acid/base where appropriate, rough quantitative measurements regarding neutralisation reactions (e.g. drop-by-drop titrations to distinguish hydrochloric acid and sulfuric acid) and the smell of different solutions- Note that the buffer solutions may change pH gradually in a similar way to some of the acid and base solutions, making it tricky to identify the buffer solutions on pH change alone. The differences in pH values will aid identification here.

Below are a few guidance points for teachers to consider:
• Solutions containing carboxylic acids and ammonia may have a detectable odour.
• The two buffer solutions can be distinguished from solutions of weak acids and bases by adding 0.5 mol dm3 HCl and/or NaOH dropwise and monitoring the change in pH. The buffer solutions should resist changes in pH as small amounts of acid and base are added.
• Learners are given the Ka values of methanoic and ethanoic acid on their worksheet, which indicates that methanoic acid is the stronger of the two. This means it would be expected to have a slightly lower pH.