CHEMISTRY · Multi-part · A Level · Chemistry · Acids and bases · Acids and bases
Problem
A student titrates a monoprotic weak acid, HA, with a standardized solution of strong base. The titration curve shows an initial rapid rise in pH followed by a buffer region and then a sharp increase near the equivalence point.
Consider the solution at the exact halfway point to the equivalence point (often called the half-equivalence point).
(a) Describe qualitatively how the relative amounts of HA and its conjugate base A⁻ change from the start of the titration to the half-equivalence point.
(b) Using principles of chemical equilibrium, explain why the measured pH of the solution at the half-equivalence point is numerically equal to the acid’s pKₐ.
(c) Identify TWO experimental assumptions that must be satisfied for the equality stated in part (b) to hold to within ±0.05 pH unit in an actual laboratory titration.