11.3

Question 1

11.3 pH – acids and buffers • To measure pH values with a pH meter and to obtain a better understanding of pH and buffer solutions.

Answer 1

Introduction
You will first prepare hydrochloric acid solutions of a range of concentrations by dilution and measure their pH values. Then, you will prepare a range of buffer solutions and measure their pH values. For both types of solutions, you will investigate how the pH changes on addition of sodium hydroxide solution.

Question 2

Aqueous hydrogen chloride, HCl(aq), 0.100 mol dm–3

Answer 2

Currently no hazard classification at this concentration

Question 3

Aqueous ethanoic acid, CH3COOH(aq), 0.400 mol dm–3

Answer 3

Currently no hazard classification at this concentration

Question 4

Aqueous sodium ethanoate, CH3COONa(aq), 0.400 mol dm–3

Answer 4

Currently no hazard classification

Question 5

Aqueous sodium hydroxide, NaOH(aq), 0.400 mol dm–3

Answer 5

WARNING
Causes skin irritation and serious eye
irritation

Question 6

Part 1 – Preparing solutions of hydrochloric acid with different pH values

Answer 6

1. Add 10 cm3 of 0.100 mol dm–3 HCl(aq) to a boiling tube and measure its pH.
2. Dilute the 0.100 mol dm–3 HCl (aq) by a factor of 10 (i.e. 1 part HCl to 9 parts H2O) to prepare a 0.0100 mol dm–3 solution. Measure its pH.
3. Prepare further solutions of HCl (aq) with concentrations of 0.00100 and 0.000100 mol dm–3, and measure the pH of each solution.
   You need to prepare at least 40 cm3 of 0.000100 mol dm–3 HCl (aq).
4. Add 1 cm3 of 0.400 mol dm–3 NaOH(aq) to 40 cm3 of 0.000100 HCl (aq). Measure the pH of the resulting solution.

Question 7

Analysis of your resultsPart 1

Answer 7

1. Calculate the pH values of each solution prepared in 1–3 above and compare your calculated values with your experimental values.
2. Explain your results in stage 4.

Question 8

Part 2 – Preparing buffer solutions and investigating their buffering properties

Answer 8

1. Prepare three buffer solutions using the following volumes:
2. Stir each solution with a glass rod to ensure that the solutions are thoroughly mixed. Make sure that you rinse the glass rod in distilled water before using with a different solution.
3. Measure the pH of each buffer solution with a pH meter.
4. Add 1 cm3 of 0.400 mol dm–3 NaOH(aq) to each buffer solution.
   Stir to mix thoroughly and measure the pH.

Question 9

Analysis of your results Part 2
1. Calculate the pH values of each buffer solution prepared above and compare your value with your experimental values.
For ethanoic acid, Ka = 1.7  10–5 mol dm–3
2. Explain your results for Stage 4 above.
3. Compare your results to Part 2, Stage 4 with Part 1, Stage 4.

Answer 9

2. The pH increases only very slightly as the CH3COOH in the buffer solution neutralises the added NaOH(aq). The lowest amount of CH3COOH in the third buffer solution is 4.0  10–3 mol of CH3COOH and the added NaOH has 4.0  10–4 mol NaOH so CH3COOH is in great excess.
   Note that in the trial results, the measured pH after addition of NaOH is slightly lower than before addition. If learners obtain similar results, this could lead to a discussion about the accuracy of the pH meter and uncertainties in the measurements, or other sources of error. As the trial results indicate, if a pH meter is used with a lower resolution than the meter used in the trial (i.e. to ±0.2 or ±0.1 pH units), it is possible that there is no detectable difference in pH before and after addition of NaOH.
3. Learners should identify that addition of NaOH(aq) to the buffer solution caused little or no change in pH, while addition of alkali to the HCl solution caused a large change in pH. The starting solutions are of similar pH, so this shows that an ethanoate buffer maintains a much more consistent pH in this range than a solution of hydrochloric acid.

Question 10

Equipment

Answer 10

The following equipment should be provided, from which each learner or group should select appropriate apparatus. Some may select alternatives.
• Safety spectacles
• Dropping pipette
• Wash bottle containing distilled or de-ionised water (about 300 cm3 will be required)
• Glass beakers (varying sizes)
• Measuring cylinders (50, 25 and 10 cm3)
• Boiling tubes
• Glass rods
• A method for recording pH.
A pH meter/probe would be ideal. Small, ‘stick pH meters’ measuring to ±0.2 or ±0.1 pH units, could be used. These are available from most suppliers and are relatively inexpensive, but would need to be tested to ensure they produce usable results.
Alternatively narrow-range pH paper can be used but several ranges will be required.