Quick answer: A strong acid ionises completely, so its pH comes straight from its concentration. This AP Chemistry lab on acids and bases pH gives the aim, the equipment, the safety points, a step-by-step method, sample results and 7 analysis questions; the answers are on a separate page and the lab sheet is a free printable PDF.
Is Ka really a constant? You measure the pH of three dilutions of ethanoic acid, calculate Ka from each, and see that Ka stays steady while the percentage ionised changes. The lab needs a pH meter and uses safe, dilute solutions.
Level: AP Chemistry · Curriculum: AP Chem Unit 8
Aim
To measure the pH of three dilutions of ethanoic acid, calculate Ka for each, and see how the percentage ionised changes with dilution.
Equipment
- 0.100 mol/dm³ ethanoic acid, 100 cm³
- Distilled water, 250 cm³
- pH meter or probe, calibrated with pH 4 and pH 7 buffers
- Three 100 cm³ beakers and one 100 cm³ volumetric flask
- 10 cm³ and 25 cm³ pipettes with filler
- Wash bottle, tissue paper
Safety
Wear eye protection. 0.100 mol/dm³ ethanoic acid is only a mild irritant, but rinse any splash on skin with water. Rinse the probe with distilled water between solutions and never store it dry.
Method
- Calibrate the pH meter using the buffers.
- Pour about 50 cm³ of 0.100 mol/dm³ acid into a beaker, insert the probe, stir gently and record the steady pH.
- Pipette 10.0 cm³ of the 0.100 mol/dm³ acid into the flask and dilute to 100 cm³ to make 0.0100 mol/dm³. Mix well.
- Pour some into a clean beaker and record its pH.
- Repeat the tenfold dilution once more to make 0.00100 mol/dm³ and record its pH.
- Rinse the probe between each solution.
Results
Sample measurements at 25 °C:
| concentration c (mol/dm³) | measured pH | [H₃O⁺] (mol/dm³) | percent ionised | Ka |
|---|---|---|---|---|
| 0.1 | 2.88 | 1.32 × 10⁻³ | 1.3 | 1.76 × 10⁻⁵ |
| 0.01 | 3.41 | 3.89 × 10⁻⁴ | 3.9 | 1.57 × 10⁻⁵ |
| 0.001 | 3.91 | 1.23 × 10⁻⁴ | 12.3 | 1.73 × 10⁻⁵ |
Analysis questions
Question 1 Hydronium concentrations
Calculate [H₃O⁺] for each pH reading.Question 2 Percent ionised
Calculate the percent ionised at each concentration and describe the trend.Question 3 Ka from each reading
Calculate Ka at each concentration using Ka = [H₃O⁺]²/(c − [H₃O⁺]).Question 4 Mean and comparison
Calculate the mean Ka and compare it with the accepted value 1.8 × 10⁻⁵.Question 5 Predicting the next dilution
Predict the pH of the next tenfold dilution (0.000100 mol/dm³) using Ka = 1.8 × 10⁻⁵.Question 6 Pipetting error
The 10.0 cm³ pipette actually delivers 9.9 cm³ in the first dilution. Explain the effect on the concentration and on the Ka calculated for that dilution.Question 7 Random error
State two sources of error in the pH readings and one improvement for each.
