The method and the results are on the acids pH buffers lab. Do the analysis yourself first, then check the answers below.
Quick answer: A Brønsted–Lowry acid donates a proton and a base accepts one. These answers go with the Chemistry 11-12 lab on acids pH buffers: the graph, the calculations, the uncertainties and the conclusion explained, for all 7 analysis questions.
Model answers follow the sample data in the results tables. Your own readings will differ a little, so judge your work by the shape of the graph, the size of the intercept difference and the clear contrast between buffer and water.
Level: Chemistry grades 11-12 (A Level · IB · Class 11-12) · Curriculum: IB R3.1
The points lie on a straight line rising from left to right. The gradient is 0.96 (close to the theoretical 1) and the y-intercept, where log ratio = 0, is 4.75.
Question 3 Finding pKa
At log ratio 0, pH = pKa, so pKa ≈ 4.75. Difference = (4.75 − 4.74) ÷ 4.74 × 100 = 0.2 %, so the data support the equation.
Question 4 Measured and calculated pH
Predicted: 4.74 + log(0.111) = 4.74 − 0.954 = 3.79; measured 3.83, so the difference is 0.04 pH units. Reasons: the activity of ions differs from concentration; the calibration uncertainty of the meter (±0.02); temperature of 20 °C instead of 25 °C where pKa is quoted; ethanoate sample slightly impure.
The ratio is unchanged, so the calculated pH stays about 4.74. But the buffer now has ten times fewer HA and A⁻, so it has a much smaller buffer capacity: the same 2.0 cm³ of acid would change the pH far more.
Question 7 Improving the lab
The smallest volume, 5.0 cm³, has the largest percentage uncertainty (±0.1 cm³ is 2 %), though this changes log ratio by only 0.009. A more important limit is the single-point calibration drift and the unstable reading; calibrate with two buffers at the working temperature, wait for a steady reading and repeat each trial to get a mean.
