Question

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

Answer 1

See this old answer here.

Explanation:

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

Thus `pH=pK_a+log_10{[[A^-]]/[[HA]]}`

And here the following equilibrium is interrogated,

`H_3CNH_2 + H_2O rightleftharpoons H_3CNH_3^(+) + HO^-`

So for the purposes of your question, `A^(-)-=H_3CNH_2`, and `HA-=H_3CNH_3^+`.

You originally had `H_3CNH_2`, at EQUIVALENCE you have `H_3CNH_3^+`. You have NOT supplied `pK_a` for `H_3CNH_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.