pH of the solution

Question

A solution contains 0.228 M C₂H₅NH₃Br and 0.126 M ethylamine , C₂H₅NH₂ .

What is the pH?

Answer 1

A mixture of C₂H₅NH₃⁺ and C₂H₅NH₂ creates a buffer b/c you have a weak base (C₂H₅NH₂) and the conjugate acid (C₂H₅NH₃⁺). To find the pH, we can use the Henderson Hasselbalch equation:

pOH = pKb + log [C₂H₅NH₃⁺] / [ C₂H₅NH₂]

Looking up Kb for C₂H₅NH₂, tabular value is given as 5.6x10^-4 (your value may be slightly different)

pKa = -log Kb = 3.25

pOH = 3.25 + log (0.288 / 0.126)

pOH = 3.25 + 0.36

pOH = 3.61

pH = 14 - pOH

pH = 14 - 3.61

pH = 10.4