Required Practical 9 (AQA A-Level Chemistry): Revision Notes

8.1.3 Required Practical 9

Aim:

Investigate how pH changes when a weak acid reacts with a strong base and when a strong acid reacts with a weak base.

pH Probe Calibration:

Method:

1. Rinse the pH probe: Thoroughly rinse the pH probe with deionised (distilled) water and gently shake to remove any excess water.
2. Calibrate with pH 7.00 buffer: Insert the probe into the standard pH 7.00 buffer solution provided.
3. Ensure the bulb is fully immersed, then record the pH reading.
4. Repeat calibration for other buffers: Rinse the probe again with deionised water before each reading.
5. Repeat the procedure using pH 4.00 and pH 9.20 buffer solutions, recording the pH values for each.
6. Plot the calibration graph: Create a graph by plotting your recorded pH readings (x-axis) against the known pH values of the buffer solutions (y-axis).

Measuring pH: Acid-Alkali Mixture

Method:

1. Prepare the burette with ethanoic acid: Rinse a burette with a 0.100 mol dm⁻³ solution of ethanoic acid, then fill the burette with the same solution.
2. Label the burette. Ensure the jet of the burette is completely filled without any air bubbles.
3. Transfer ethanoic acid: Use the burette to transfer exactly 20.0 cm³ of ethanoic acid into a clean 100 cm³ beaker.
4. Prepare the burette with sodium hydroxide ( $NaOH$ ): Rinse a second burette with 0.100 mol dm⁻³ $NaOH$ solution, then fill this burette with the same $NaOH$ solution.
5. Label the second burette, ensuring the jet is filled without air bubbles.
6. Set up the pH probe: Rinse the pH probe with distilled (deionised) water and clamp it so that the bulb is fully immersed in the ethanoic acid solution in the beaker.
7. Stir and record the initial pH: Stir the solution gently with a glass rod and record the initial pH reading in a suitable table.
8. Add $NaOH$ in increments: Add 2.0 cm³ of $NaOH$ solution from the burette at a time, stirring the solution and recording the pH after each addition.
9. Record the pH and the corresponding volume of $NaOH$ added.
10. Approach the endpoint: As the endpoint approaches, reduce the increments of $NaOH$ added to 0.20 cm³, continue to stir the solution and record the pH and volume of $NaOH$ added after each addition.
11. Complete the titration: After reaching the endpoint, continue adding 2.0 cm³ portions of $NaOH$ until an excess is achieved.
12. Stir the solution and record the pH and volume for each addition. These results will allow a pH curve to be plotted.

Analysing the Data:

• Use the pH probe calibration graph to correct the pH readings obtained during the experiment. These corrected pH values should be recorded in a new column in the results table.
• Plot a graph of the corrected pH values (y-axis) against the volume of sodium hydroxide ($NaOH$) solution added (x-axis).
• Connect the data points in the most appropriate way, forming a smooth curve.