A 28.5 mL sample of 0.393 M trimethylamine, (CH3)3N, is titrated with 0.273 M hydrobromic acid.

Question

A 28.5 mL sample of 0.393 M trimethylamine, (CH₃)₃N, is titrated with 0.273 M hydrobromic acid.

The pH before the addition of any hydrobromic acid is______

Answer 1

The pH before the addition of HBr is determined from the hydrolysis of the weak base (CH3)3N. To find the pH, we need to know the Kb for (CH₃)₃N. Looking it up in a table of Kb values, it is found to be 4.38x10^-4

(CH₃)₃N + H2O ==> (CH₃)₃NH⁺ + OH^-

Kb = 4.38x10^-4 = [(CH₃)₃NH⁺] [OH^-] / [(CH₃)₃N]

4.38x10^-4 = (x)(x) / 0.393

x² = 1.72x10^-4

x = 0.0131 M = [OH^-]

pOH = 1.88

pH = 14 - 1.88

pH = 12.12