Question

Consider a buffer solution formed from 0.1M HF and 0.1M NaF.

1) If a compound containing H+ is added to this solution, what direction will the equilibrium reaction shift and what species is consumed in the process?

2) Why is it necessary, then, to have NaF as well as HF present for this solution to behave as a buffer?

Answer 1

1) NaF is strong electrolyte and hence, it will be dissociated completely in Na+ and F-

NaF -------> Na+ + F-

While HF is weak acid and it will be dissociated partially.

HF -------> H+ + F-

It means concentration of F- will be high in the solution (product side has high concentration) . So, when we add H+ ion in the solution, H+ ions will react with F- to form HF i.e. all H+ will be consumed by F- ions. Thus, equilibrium will be shifted to reactants side. And F- ions will be consumed by H+ ions

2) Due to common ion effect, the concentration of F- ions will be consumed by H+ ion if we add acid to the solution. And, hence concentration of H+ will not be increased. Thus, pH of solution will not change.