If p, q and r are the pK_b values of methylamine, N-methylamine and N,N-dimethylamine respectively, what is the correct order of p, q and r?

r > p > q

This peculiar order of basic strength is due to a combination of inductive, solvation and stearic effects of the alkyl groups in the amines. In methyl substituted amines, the solvation effect superimposes the inductive effect to make CH₃NH₂ more basic than (CH₃)₃N. The 2° compound is the most basic because of its higher inductive effect than primary amine and higher solvation than tertiary amine.