A chemist mixes 25.0 mL of 0.040 M acetic acid, CH₃COOH(aq)
CHEMISTRY · Multi-part · A Level · Chemistry · Acids and bases · Acids and bases
Problem
A chemist mixes 25.0 mL of 0.040 M acetic acid, CH₃COOH(aq), with 25.0 mL of 0.020 M hydrochloric acid, HCl(aq), to create the analyte for a subsequent titration with NaOH. The temperature is 25 °C and the acid-dissociation constant of acetic acid is K_a = 1.8 × 10⁻⁵.
(a) Calculate the concentration of hydronium ions that arises solely from the complete dissociation of HCl in the combined solution.
(b) Taking the value from part (a) into account, determine the equilibrium concentration of hydronium ions after acetic acid establishes equilibrium with water. Show all steps in your reasoning.
(c) Compute the initial pH of the mixed solution based on your work in parts (a) and (b).
(d) Evaluate whether ignoring the contribution of acetic acid to [H₃O⁺] would introduce a significant error under these conditions. Support your answer quantitatively.