Chemistry Question

Question

Consider an aqueous solution of ammonia, NH3, a base. If the resulting pH of this solution is 9.5, then what is the concentration of this ammonia solution? Note that the Ka for ammonium, NH4, is 5.56 x 10^(-10)

Answer 1

Kw = 1x10^-14 = KaKb

Kb = 1x10^-14/5.56x10^-10 = 1.8x10^-5

NH3 + H2O ==>NH4OH ==> NH4⁺ + OH^-

Kb = [NH4+][OH-]/[NH3]

pH = 9.5

pOH = 14-9.5 = 4.5

[OH-] = 1x10^-4.5 = 3.16x10^-5

1.8x10^-5 = (3.16x10^-5)²/[NH₃]

[NH3] = (3.16x10^-5)²/1.8x10^-5 

[NH3] = 5.5x10-5 M