Question

What is the pH of a 0.240 M solution of ammonium formate (NH₄HCOO)? The K_A for NH₄⁺ is 5.6E-10 and the K_B for HCOO^- is 5.6E-11.

Answer 1

the salt is from weak acid and weak base . so the pH depends on pKa and pKb vlaues of acid and base . but not concentration of the salt

Kb of NH3 = Kw / Ka = 1.0 x 10^-14 / 5.6 x 10^-10

Kb = 1.79 x 10^-5

pKb = 4.75

Ka of acid = Kw / Kb

                   = 1.0 x 10^-14 / 5.6 x 10^-11

                   = 1.79 x 10^-4

Ka = 1.79 x 10^-4

pKa = 3.75

pKa < pKb . so the salt is acidic

pH = 7 + 1/2 [pKa - pKb]

pH = 7 + 1/2 [3.75 - 4.75]

pH = 6.5