What is the pH of a 7.0×10−3 M NH3 solution? (pKb = 4.74)

Question

Can someone plz explain this to me plz?

Answer 1

Looking at the reaction when NH3 dissolves in water:

NH₃ + H₂O ==> NH₄⁺ + OH^-

Kb = [NH4⁺][OH^-] / [NH3] and we can find Kb from pKb to be = 1.82x10^-5

1.82x10^-5 = (x)(x) / 7.0x10^-3 - x

x² = 1.27x10^-7 - 1.82x10^-5x

x = 3.47x10^-4 M = [OH^-]

pOH = -log 3.47x10^-4 = 2.46

pH = 14 - 2.46

pH = 11.5