What is the pH of a solution of 0.400 M CH₃NH₂ containing 0.180 M CH₃NH₃I? (Kb of CH₃NH₂ is 4.4 × 10⁻⁴)

Answer 1

This solution contains a weak base (CH3NH2) and the conjugate acid of that weak base (CH3NH3I).

This makes a BUFFER. To find the pH of a basic buffer, we can use a modification of the Henderson Hasselbalch equation. We can use pOH = pKb + log [salt] / [base]

pKb = -log Kb = -log 4.4x10^-4

pKb = 3.36

pOH = 3.36 + log (0.180/0.400)

pOH = 3.36 + log 0.45

pOH = 3.36 + (-0.35)

pOH = 3.71

pH + pOH = 14

pH = 14 - pOH

pH = 10.29