One equivalent of dilute HCl is added to an equimolar aqueous mixture of CH3NH2 (pKaH 10.6) and C6H5NH2 (pKaH 4.6), the amounts of HCl and of each amine being equal. The main species present are

equal amounts of CH3NH3+ and C6H5NH3+

CH3NH2 and C6H5NH2 only, with free HCl

JEE Advanced levelSingle correct optionDifficulty 3 of 3g2098

One equivalent of dilute HCl is added to an equimolar aqueous mixture of $CH X_{3} NH X_{2}$ (pKaH 10.6) and $C X_{6} H X_{5} NH X_{2}$ (pKaH 4.6), the amounts of HCl and of each amine being equal. The main species present are

A $CH X_{3} NH X_{2}$ and $C X_{6} H X_{5} NH X_{3} X^{+}$
B $CH X_{3} NH X_{3} X^{+}$ and $C X_{6} H X_{5} NH X_{2}$
Cequal amounts of $CH X_{3} NH X_{3} X^{+}$ and $C X_{6} H X_{5} NH X_{3} X^{+}$
D $CH X_{3} NH X_{2}$ and $C X_{6} H X_{5} NH X_{2}$ only, with free HCl

The stronger base takes the proton first. The equilibrium

CH X_{3} NH X_{2} + C X_{6} H X_{5} NH X_{3} X^{+} CH X_{3} NH X_{3} X^{+} + C X_{6} H X_{5} NH X_{2}

has

K = 1 0^{10.6 - 4.6} = 1 0^{6}

, so essentially all the HCl protonates methylamine, leaving aniline unprotonated.