PAG 11

Question 1

1. Calculate the expected pH values of the different solutions.

Answer 1

check sheet

Question 2

11.1 equipement

Answer 2

• safety goggles
• chemical-resistant gloves
• a method for recording pH
Ideally a pH probe, or meter. Small, ‘stick’ pH meters could be used, but should be tested to see if they provide usable data. These are available from most suppliers and are relatively inexpensive.
Alternatively narrow-range pH paper can be used but several ranges will be required.
• distilled water in a wash bottle
• several plastic dropping pipettes
• suitable containers if using pH probes/meter

Question 3

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 3

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 4

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 4

. 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 5

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 5

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

Question 6

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

Answer 6

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 7

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

Answer 7

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