A buffered aqueous solution is prepared by combining solid sodium acetate and glacial…
CHEMISTRY · Multi-part · AP · Chemistry · Acids and Bases · Acids and Bases
Problem
A buffered aqueous solution is prepared by combining solid sodium acetate and glacial acetic acid such that the final pH is maintained at 5.00 with a total buffer concentration (acid + conjugate base) of 0.50M. Solid lead(II) fluoride, PbF2, is then added in excess, and the system is allowed to reach equilibrium at 25 °C. The following data are available.
Ksp[PbF2] = 3.3×10−8
Ka[HF] = 6.8×10−4
Assume that the buffer capacity is large enough so that the pH remains fixed at 5.00, the volume change upon adding solids is negligible, and no other complex ions form.
(a) Calculate the equilibrium concentration of hydronium ion, [H+], in the buffered solution.
(b) Write the chemical equilibria that are relevant to the dissolution of PbF2 in this buffered environment, including any protonation reactions of the fluoride ion.
(c) Derive a mathematical expression for the molar solubility s of PbF2 in the pH 5.00 buffer in terms of Ksp, Ka, and [H+]. State any approximations you invoke.
(d) Calculate the numerical value of s under these conditions and determine by what factor the solubility is altered relative to its value in pure water at the same temperature.