Quick answer: A buffer is a solution of a weak acid and its conjugate base that resists pH change. This AP Chemistry lab on buffer solutions 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.
Where is the equivalence point, and what does the curve tell you about the acid? You titrate ethanoic acid with sodium hydroxide while a pH meter records the change, then find the concentration, the pKa and the best indicator from your graph.
Level: AP Chemistry · Curriculum: AP Chem Unit 8
Aim
To plot the pH curve for the titration of ethanoic acid with sodium hydroxide, find the equivalence volume, determine pKa from the half-equivalence point and choose an indicator.
Equipment
- 0.100 mol/dm³ NaOH in a 50 cm³ burette
- 25.0 cm³ of ethanoic acid of unknown concentration
- pH meter and probe, calibrated with pH 4 and pH 7 buffers
- Conical flask, 25.0 cm³ pipette, funnel, stand, white tile
- Distilled water, eye protection
Safety
Wear eye protection. Sodium hydroxide at 0.1 mol/dm³ is irritant to eyes and skin, so wash splashes with plenty of water at once. Mop up spills straight away and fill the burette below eye level.
Method
- Rinse the burette with NaOH and fill it. Record the starting reading.
- Pipette 25.0 cm³ of the acid into the flask and add the pH probe.
- Record the initial pH.
- Add NaOH in 5 cm³ portions, swirling and recording pH each time, then in 1 cm³ portions near the steep rise.
- Add 0.1 cm³ portions around equivalence, then continue to 30 cm³.
- Plot pH against volume.
Results
Sample readings for 25.0 cm³ of acid with 0.100 mol/dm³ NaOH:
| volume NaOH (cm³) | pH |
|---|---|
| 0 | 2.88 |
| 5 | 4.14 |
| 10 | 4.57 |
| 12.5 | 4.74 |
| 15 | 4.92 |
| 20 | 5.35 |
| 24 | 6.12 |
| 24.9 | 7.14 |
| 25 | 8.72 |
| 25.1 | 10.30 |
| 26 | 11.29 |
| 30 | 11.96 |
Analysis questions
Question 1 Equivalence volume
Use the steepest part of the data to state the equivalence volume.Question 2 Concentration of acid
Calculate the concentration of the ethanoic acid.Question 3 Finding pKa
State the half-equivalence volume and read pKa from the data.Question 4 Ka value
Calculate Ka from your pKa and compare with 1.8 × 10⁻⁵.Question 5 Indicator choice
The equivalence pH is about 8.7. Choose between methyl orange (3.1–4.4), bromothymol blue (6.0–7.6) and phenolphthalein (8.3–10.0).Question 6 Buffer region
Explain why the pH changes slowly between 5 cm³ and 20 cm³.Question 7 Uncertainty
The burette is read to ±0.05 cm³ at start and finish. Calculate the percentage uncertainty in the equivalence volume of 25.0 cm³.
