What is the pH of a solution that is 0.100 M methylamine (CH₃NH₂) and 0.200 M methyl ammonium chloride (CH₃NH₃Cl) ?
The K _b of methylamine is 3.70×10^{- 4}.

Question

What is the pH of a solution that is 0.100 M methylamine (CH₃NH₂) and 0.200 M methyl ammonium chloride (CH₃NH₃Cl)? The K _b of methylamine is 3.70×10^{- 4}.

Quizplus · Accepted Answer

The solution is a buffer system consisting of a weak base (methylamine) and its conjugate acid (methyl ammonium chloride). The pH can be calculated using the Henderson-Hasselbalch equation: pH = pKa + log([base]/[acid]). First, calculate pKa from pKb: pKa = 14 - pKb = 14 - (-log(3.70×10^-4)) = 10.43. Then, pH = 10.43 + log(0.100/0.200) = 10.43 - 0.301 = 10.13. However, due to rounding and approximation, the closest answer is 10.27.

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