Question

Calculate the pH of:
a) ammonia, 0.1M, in water
b) ammonium chloride, 0.1M, in water
c) a buffer solution containing ammonia (0.1M)and ammonium chloride (0.01M)

For NH4^+, pKa = 9.25

Answer 1

a)
NH3 + H2O ==> NH4 + OH^-

Kb = (NH4^+)(OH^-)/(NH3)

(NH4^+) = y
(OH^-) = y
(NH3) = 0.1 - y
Plug into Kb and solve for y.

b) NH4Cl + HOH ==> NH4^+ + Cl^-
The NH4^+ hydrolyzes as follows:
NH4^+ + H2O ==> NH3 + H3O^+
Ka = Kw/Kb = (NH3)(H3O^+)/(NH4^+)
(NH3) = y
(H3O^+) = y
(NH3) = 0.1 - y
solve for y.

c) Use the Henderson-Hasselbalch equation.

Post your work if you get stuck on any of these.