A laboratory technician prepares 100 mL of an aqueous buffer by mixing 0.30 M acetic…
CHEMISTRY · Multi-part · A Level · Chemistry · Acids and bases · Acids and bases
Problem
A laboratory technician prepares 100 mL of an aqueous buffer by mixing 0.30 M acetic acid (CH₃COOH, pKₐ = 4.74) with 0.20 M sodium acetate (CH₃COONa).
Assume that activity coefficients remain unity throughout the experiment.
(1) Identify which component of the conjugate-acid–base pair is present in stoichiometric excess in the prepared buffer and briefly justify your answer using the given molarities.
(2) Predict qualitatively whether the pH of the buffer will increase, decrease, or remain essentially unchanged after the addition of 2.0 mL of 1.0 M NaOH. No numerical pH calculation is required; support your prediction with chemical reasoning.
(3) Predict qualitatively the direction of pH change when 10.0 mL of the same NaOH solution is added to another 100 mL aliquot of the buffer. Explain how this larger addition relates to the concept of buffer capacity.
(4) Explain why the responses in parts (b) and (c) differ, citing the concentration effect and the component-ratio effect described in buffer theory.