When finding pH of acidic buffer solution using Ka, why do we use the salt concentration in the Ka equation?

Question

for example when CH3COOH and CH3COONa are used to make a buffer solution, why we use the [CH3COO-] of the salt in the Ka equation of the acid? isn't the [CH3COO-] from acid not the same concentration of that from salt?

Answer 1

Hello, Shrooq,

The weak acid, CH3COOH (acetic acid), dissociates into the acetate ion, CH3COO^- and a hydrogen ion, H⁺. As far as they are concerned, their solubility constant holds true, regardless of where any additional acetate or hydrogen ions may come from. When sodium acetate is added, the CH3COO^- ions from it look, and are, the same as the ones originally from the intact acid. The equilibrium is thus disturbed. The additional acetate ions pressure the equilibrium to go in reverse: they will soak up some of the H+ ions and become acetic acid, until the product of their concentrations equals the K_sp. That will have the effect of reduced the H⁺ concentration, and thus raise the pH.

Bob

Answer 2

Because [CH3OO-] from the CH3COONa is going to be way bigger than [CH3COO-] from the dissociation of CH3COOH.