Question

(CH3)2NH is more basic than (CH3)3N in an aqueous solution.. Why.???

Answer 1

With increase in alkyl group the +l effect will increase which will increase the ease of donation of lone pair electron.But in water one other factor is controlling the strength of basicity.
Amine will accept a proton & form cation.THIS CATION WILL BE STABILISED IN WATER BY SALVATION(by hydrogen bonding).Better the salvation by hydrogen bonding,higher will be the basic strength.
Thus with increase in methyl group,hydrogen bonding & stabilisation by salvation decreases.The net effect is when we move from secondary to tertiary amine basic strength actually decreases.

Answer 2

(CH3)2NH is secondary amine and (CH3)3N is tertiary amine. Therefore secondary amine is more basic then tertiary. It depends upon the no. of carbon atoms attached on nitrogen