NH3 + H20--> NH4+ + OH-?

Question

Ammonia is a weak base that dissociated in water, as shown above. At 25°C, the base dissociation constant, Kb, for NH3 is 1.8 x 10-5.

a. Determine the hydroxide ion concentration and the percentage dissociation of a 0.150 molar solution of ammonia.

b. Determine the pH of a solution prepared by adding 0.0500 mole solid ammonium chloride to 100. milliliters of 0.150 molar solution of ammonia.

Answer 1

1.8 x 10^-5 = x^2 / 0.150 -x

x = [OH-] =0.00164 M

% = 0.00164 x 100 / 0.150 = 1.10

[NH4+] = 0.0500 mol / 0.100 L = 0.500

pKb = 4.7

pOH = 4.7 + log 0.500 / 0.150 = 5.2

pH = 14 - 5.2 =8.8