A chemist prepares two fluoride buffer suspensions at 25∘C. In both cases the solvent volume is initially 500mL.
Buffer E: [HF]=0.30M and [F−]=0.30M.
Buffer F: Identical to Buffer E, but an excess of solid CaF2 is added and remains in contact with the solution. The solubility‐product constant of calcium fluoride is Ksp=3.9×10−11 at 25∘C.
For hydrofluoric acid, Ka=6.8×10−4.
(a) Write balanced chemical equations for (i) the acid–base equilibrium of the HF/F− pair and (ii) the dissolution equilibrium of CaF2(s).
(b) Demonstrate quantitatively that the presence of solid CaF2 constrains the equilibrium fluoride-ion concentration in Buffer F to a value determined by Ksp.
(c) Calculate the initial pH of Buffer E.
(d) A 0.010mol aliquot of HCl(aq) is introduced into each buffer. Assuming volume change is negligible and that CaF2 remains present in Buffer F, determine the pH of each resulting mixture.
(e) Using your calculations, compare the capacities of Buffers E and F to resist the added acid. Account for any difference in performance by referencing the equilibria written in part (a).