Chemistry pH question

Question

 is a weak base , so the salt  acts as a weak acid. What is the pH of a solution that is  in  at ?

Answer 1

Let CH3NH2 be represented by B

B + HNO₃ ==> BH⁺ + NO₃^-

BH⁺ ==> B + H⁺ Ka = 1x10^-14/5.0x10^-4 = 2.0x10^-11

Ka = 2.0x10^-11 = [B][H⁺]/[BH⁺]

2.0x10^-11 = (x)(x)/(0.0850 - x) and assuming x is small compared to 0.0850 we can ignore it.

2.0x10^-11 = x²/0.0850

x² = 1.7x10^-12

x = 4.12x10^-7 = H⁺ (note: this value is much lower than 0.0850 so ignoring x above was justified)

pH = -log [H⁺] = -log 4.12x10^-7

pH = 6.39