# What is pH when HCl(aq) is added to an aqueous solution of "methylamine"?

Apr 28, 2017

#### Explanation:

The defining relationship is.............

Thus $p H = p {K}_{a} + {\log}_{10} \left\{\frac{\left[{A}^{-}\right]}{\left[H A\right]}\right\}$

And here the following equilibrium is interrogated,

${H}_{3} C N {H}_{2} + {H}_{2} O r i g h t \le f t h a r p \infty n s {H}_{3} C N {H}_{3}^{+} + H {O}^{-}$

So for the purposes of your question, ${A}^{-} \equiv {H}_{3} C N {H}_{2}$, and $H A \equiv {H}_{3} C N {H}_{3}^{+}$.

You originally had ${H}_{3} C N {H}_{2}$, at EQUIVALENCE you have ${H}_{3} C N {H}_{3}^{+}$. You have NOT supplied $p {K}_{a}$ for ${H}_{3} C N {H}_{3}^{+}$, and you are going to need this. Remember that as you add titrant, the volume of the solution is going to change, as is the molar concentrations of the reagents.