Determine the pH of a buffer which is a 0.20 M solution of trimethylamine (N(CH3)3) and a 0.40 M solution of trimethylammonium chloride (NH(CH3)3Cl). The Kb of trimethylamine at 25°C is 6.3x10-5.

pH of the buffer is 10.10

Explanation:

trimethylamine is a weak base that, in presence with its conjugate base, trimethylammonium ion, produce a buffer.

To determine the pH of the buffer we use H-H equation for weak bases:

pOH = pKb + log [Conjugate acid] / [Weak base]

pKb is -log Kb = 4.20

pOH = 4.20 + log [N(CH₃)₃] / [NH(CH₃)₃]

Replacing the concentrations of the problem:

pOH = 4.20 + log [0.20M] / [0.40M]

pOH = 3.90

As pH = 14 -pOH