Question

What is the pH of a solution in which 0.10 mole of NH3 (pKb = 1.8 x 10^-5) is mixed with 0.10 mole of acetic acid (pKa = 1.8 x 10^-5)?

a. 4.74

b. 3.74

c. 7.0

d. 9.26

e. 8.26

Answer 1

Answer 'C'

CH₃COOH + NH₃ -----------------> CH₃COONH₄

[NH₃] = [CH₃COOH] = 0.10                K_a = K_b = 1.8 x 10^-5

As both CH₃COOH, NH₃ are weak acid and weak base respectively,,

[H⁺] = = = 1.34 x 10^-3

[OH^-] = = = 1.34 x 10^-3

1 mole of CH₃COOH reacts with 1 mole of NH₃ to give 1 mole of CH₃COONH₄

1.34 x 10^-3 mole of CH₃COOH reacts with 1.34 x 10^-3 mole of NH₃ to give 1.34 x 10^-3 mole of CH₃COONH₄

The solution containing salt of weak acid and weak base (CH₃COONH₄)

Salt hydrolysis takes place

Since P^K_a=P^K_b, P^H=7

Hence, Solution will be neutral in nature.