NH3 has a pKb of 4.74. What is the pH of a 3.2 x 10-4 mol L-1 NH3 solution?

Question

I did this question 4 times and got it wrong, I keep getting 13 but the correct answer is 9.83, can someone please explain this to me?

Answer 1

NH3 + H20 == NH4 + OH

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

pKb = 4.74 ... Kb = 1.82x10^-5

1.82x10^-5 = (x)(x)/3.2x10^-4

x² = 5.84 x10^-9

^{x = 7.6x10-5 = [OH-]}

^{pOH = 4.1}

^{pH = 14 -4.1 = 9.9}