Acid based properties

Question

NH3 is a weak base ( 𝐾b=1.8×10−5 ) and so the salt NH4Cl acts as a weak acid. What is the pH of a solution that is 0.045 M in NH4Cl at 25 °C?

pH= ?

Answer 1

NH₄⁺ ==> NH₃ + H⁺ .. hydrolysis of NH₄⁺

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

5.56x10^-10 = [NH₃][H⁺] / [NH₄⁺] = (x)(x) / 0.045 - x (assume x is small relative to 0.045 and ignore it)

x² = 2.5x10^-11

x = 5x10^-6 M = [H⁺] (note: this is less than 1.5% of 0.045 so above assumption was valid)

pH = -log [H+] = -log x = 5x10^-6 M

pH = 5.3