Question

a)Consider an aqueous solution of ammonium chloride, NH4Cl. Neglecting the autoprotolysis of water, calculate the pH of NH4Cl for formal concentration of 0.10M.

b)Use the systematic treatment of equilibrium to derive an equation for an aqueous solution of NH4Cl that contains [H+], F, Ka and Kw. Do not neglect the autoprotolysis of water in this case.

Answer 1

a) NH4+ + H2O--------------------------> NH4OH + H+

0.1                                              0             0

-x                                               x               x

0.1-x                                          x               x

Ka = [NH4OH][H+]/[NH+] = x^2/0.1-x

1.8 x10^-6= x^2/0.1-x

x= 7.4 x10^-6

[H+]= x = 7.4 x10^-6 M

pH= -log[H+] = -log(7.4 x10^-6)= 5.13

pH= 5.13

b) NH4Cl ------------------------------> NH4+ + Cl-

NH4+ + 2H2O -------------------------> NH4OH + H3O+ (salt hydrolysis + auto protolysis of water )

   Ka= [ NH4OH ][H3O+]/[NH4+]

Kw/Kb= [ NH4OH ][H3O+]/[NH4+]