What is NH4+ concentration of the 3.0M ammonia solution below?

Question

Acid: HF 

Conjugate base: NH3

Ka: 7.2×10^-4

pKa: 3.14

 

Kb: 1.8×10^-5

 

pKb: 4.74

Answer 1

This is from the previous question that I answered as follows:

NH3 + H2O <==> NH4+ + OH-
Kb = [NH4+][OH-]/[NH3]
1.8x10-5 = (x)(x)/3.0
x2 = 5.4x10-5
x = 7.35x10-3 = [OH-]

7.35x10-3 is not only the [OH-], it is also the [NH4+] as seen from the dissociation equation.