RP9 - Investigate how pH changes when a weak acid reacts with a strong-base and when a strong acid reacts with a weak base

Question 1

Ph Probe Calibration

Question 2

Describe how this can be carried out?

Answer 2

1. Rinse the pH probe thoroughly with deionised (distilled) water and shake gently to remove excess water.
2. Place the probe in the standard pH 7.00 buffer solution provided. Record the pH reading. Ensure the bulb is fully
   immersed.
3. Repeat this process using the standard pH 4.00 and pH 9.20 buffer solutions. Record the pH readings. Rinse the pH probe thoroughly with deionised water before taking each reading.
4. Plot a graph of your recorded pH reading (x-axis) against the pH of the buffer solution (y-axis).

Question 3

How pH changes when a weak acid reacts with a strong base

Question 4

Describe how this reaction can be carried out?

Answer 4

1. Rinse a burette with 0.100 moldm-3 solution of ethanoic acid and then fill the burette with this solution. Label this burette. Ensure the tap of the burette is filled without any air bubbles.
2. Use the burette to transfer exactly 20.0 cm3 of ethanoic acid to a clean 100 cm3 beaker.
3. Rinse a second burette with 0.100 mol dm-3 NaOH solution and then fill this second burette with this solution. Label this burette. Ensure the jet of the burette is filled without
   any air bubbles.
4. Rinse the pH probe with distilled (deionised) water and clamp it so that its bulb is fully immersed in the ethanoic acid solution in the beaker.
5. Use a rod to stir the solution gently and record the pH reading in a suitable table
6. Add 2.0 cm3 of NaOH solution from the burette at a time. Stir the solution and record the pH alongside the volume of NaOH added.
7. When the end-point is being reached, add 0.20 cm3 of NaOH each time. Stir the solution and record the pH alongside the volume of NaOH added.
8. After this, continue adding 2.0 cm3
   NaOH until it is in excess. Stir the solution
   and record the pH alongside the volume of NaOH added.

Question 5

Draw a diagram to show this experimental set up

Question 6

How can we analyse the data?

Answer 6

● Use the pH probe calibration graph to adjust the pH readings obtained in the experiment. These corrected pH values should be entered into a new column in the table of results.
● Plot a graph of the corrected pH values (y-axis) against volume of sodium hydroxide solution added.
● Join the points in the most appropriate way (should be a curve).