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

1 Answer
Apr 28, 2017

Answer:

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.