Calculate the Ka of NH4+

Question

a) Calculate the K_a of NH₄⁺

b) What is the pH of a 3.172 M NH₄Br solution?

Answer 1

(a). How can we calculate the Ka for NH₄⁺ without any data?

(b). NH₄Br + H₂O ==> NH₄OH + HBr The pH will be acidic. To calculate pH, we look at hydrolysis of NH₄⁺

NH₄⁺ + H2O ==> H3O⁺ + NH₃ (NH₄⁺ is acting as an acid). Looking up Ka we find Ka = 5.6x10^-10

Ka = [NH₃][H3O⁺] / [NH₄⁺] = (x)(x) / 3.172-x but ignore x in denominator as it is small

5.6x10^-10 = x² / 3.172

x² = 1.78x10^-9

x = 4.22x10^-5 = [H3O⁺]

pH = -log 4.22x10^-5

pH = 4.37