You have 600 mL of 50 mM methyl amine (pKa=10.7). Using a pH meter, you add HCl

Question

You have 600 mL of 50 mM methyl amine (pKa=10.7). Using a pH meter, you add HCl until the pH is 10.4, than add water to a total volume of 1 liter. What are the concentrations of methyl amine base and its conjugate acid in the final solution?

Explanation / Answer

600ml 50mM methyl amine = 50x10^ -3 x600/1000 = 0.03 methyl amine + H+ ---> conjugate acid 0.03-x x pkb = 3.36 Kb = [conjugate acid]/[base] 10^ -3.36 = x/(0.03-x) 0.0004365 = x/(0.03-x) x = 0.0000131 = 1.31 10^ -5 moles vol = 1.6 liter [methyl amine] = 0.03-0.0000131/1.6 = 0.01874 M [conjugate acid] = (0.0000131/1.6) = 8.187 x10^ -6 M

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