Question asking for pH of Ammonia?

Question

The pH of a solution of 0.066 mol/L ammonia is?

Answer 1

Ammonia (NH₃) is a weak base, which means it is a proton (H⁺) acceptor. When added to water, it forms ammonium and hydroxide ions. The ionization reaction can be written as:

NH₃ (aq) + H₂O (l) ⇔ NH₄⁺ (aq) + OH^- (aq)

The K_b expression is:

K_b = [NH₄⁺] [OH^-] / [NH₃] = 1.8 x 10^-5

Setup an ICE table to determine the concentrations of NH₄⁺ and OH^-.

[NH₃]_i = 0.066 mol/L

[NH₃]_eq = 0.066 - x ==> Assume x is negligible.

[NH₄⁺]_eq = x

[OH^-]_eq = x

Substitute the above values in the K_b expression, you get

1.8 x 10^-5 = (x) (x) / 0.066

x² = 1.8 x 10^-5 x 0.066

x² = 1.2 x 10^-6

x = √1.2 x 10^-6 = 1.1 x 10^-3

[OH^-] = 1.1 x 10^-3 mol/L

pOH = -log [OH^-] = -log 1.1 x 10^-3 = 2.96

pH + pOH = 14.00

pH = 14.00 - pOH = 14.00 - 2.96 = 11.04