Question

Equal volumes of 0.328 M aqueous solutions of formic acid (HCOOH(aq)) and sodium acetate (NaCH3COO) are mixed.

(1) Write the net ionic equation for the overall reaction that takes place as the system comes to equilibrium. Write acetic acid, benzoic acid or formic acid and their conjugates in the form RCOOH / RCOO-. For example, benzoic acid should be written "C6H5COOH" NOT "C6H5CO2H". It is not necessary to include states such as (aq).

(2) What is the value of the equilibrium constant for this reaction?

(3) What is the pH of the resulting solution?

Answer 1

Sol :-

(a). The partial dissociation of HCOOH is :

HCOOH (aq) + H₂O (l) <-----------------> HCOO^- (aq) + H₃O⁺ (aq), K_a = 1.8 x 10^-4 ...........(1)

Partial dissociation of CH₃COONa is :

CH₃COO^- (aq) + H₂O (l) <------------------> CH₃COOH (aq) + OH^- (aq), K_b = 5.55 x 10^-10 ...........(2)

Also,

H₃O⁺ (aq) + OH^- (aq) <--------------------> H₂O (l) + H₂O (l) , K' = 1/ K_w = 1/1.0 x 10^-14 ...........(3)

Adding equations (1), (2) and (3), we have balance net ionic equation is :

HCOOH (aq) + CH₃COOH (aq) <------------------> HCOO^- (aq) + CH₃COOH (aq)

(b) . Equilibrium constant = K = K_axK_b x 1/K_w = 9.99 = 10