The method and the results are on the buffer solutions lab. Do the analysis yourself first, then check the answers below.
Quick answer: A buffer is a solution of a weak acid and its conjugate base that resists pH change. These answers go with the AP Chemistry lab on buffer solutions: the graph, the calculations, the uncertainties and the conclusion explained, for all 7 analysis questions.
Here are the buffer solutions lab answers for AP Chemistry: graph, calculations, uncertainties and conclusion. First do the lab yourself, then compare, and go back to the lesson if needed.
The sample data were taken at 25 °C with a calibrated meter. Your results will vary slightly with the probe and the exact concentrations. Use the same graph for all the answers and show every substitution.
Half of 25.0 cm³ is 12.5 cm³, where the pH is 4.74. At this point [HA] = [A⁻], so pKa = 4.74.
Question 4 Ka value
Ka = 10⁻⁴·⁷⁴ = 1.82 × 10⁻⁵. This differs from 1.8 × 10⁻⁵ by 1 %, within experimental error.
Question 5 Indicator choice
Phenolphthalein changes colour between pH 8.3 and 10.0, inside the steep part (7 to 10.3) and around the equivalence pH. Phenolphthalein.
Question 6 Buffer region
In this region the flask contains both CH₃COOH and CH₃COO⁻, a buffer. Added OH⁻ converts HA to A⁻ and the ratio changes slowly, so the pH changes by only about 1.2 units (from 4.14 to 5.34).
