A 25.00 mL sample of an unknown monoprotic weak acid of initial concentration is…
CHEMISTRY · Multi-part · A Level · Chemistry · Acids and bases · Acids and bases
Problem
A 25.00 mL sample of an unknown monoprotic weak acid of initial concentration C0 is titrated at 25 °C with 0.100 M NaOH. The following experimental data are collected as the titration proceeds:
Volume of NaOH added (mL)
Measured pH
25.00
4.25
40.00
4.75
At both points, the only acid–base conjugate pair present in appreciable amounts is HA/A⁻. No additional buffering species are introduced.
Use the data to characterize the acid.
(a) For each data point, calculate the number of millimoles of HA converted to A⁻ during the titration. Clearly indicate the amounts of HA and A⁻ present at each stage.
(b) Using the species amounts from part (a), determine the numerical value of (pH−pKa) at 25.00 mL and at 40.00 mL of added base. Show all steps in your reasoning.
(c) Combine your results from part (b) to calculate a single, best‐estimate value of pKa for the unknown acid. Explain your method and discuss any assumptions made.
(d) Using the calculated pKa and the titration data at 25.00 mL, determine the initial concentration C0 (in mol L⁻¹) of the acid in the original sample. Provide a step‐by‐step calculation.