For the same alkyl group, all the three primary, secondary and tertiary amines are more basic than ammonia in aqueous solutions. Their relative basicity, however, depends upon a combination of three factors: +I-effect of alkyl groups, the stability of the ammonium cation (formed after accepting a proton) due to H-bonding with water molecules and the steric effect of the alkyl groups which tend to reduce the extent of H-bonding. All these three factors are favourable for secondary amines thereby making them the strongest bases. Since methyl group has the smallest size, there is no steric hindrance to H-bonding. Consequently, stability of the ammonium cation due to H-bonding with water predominates over the +I-effect of the methyl group thereby making methylamine more basic than trimethylamine. If, however, the alkyl group is bigger than the methyl group, +I-effect of the alkyl group outweighs stability of the ammonium cation due to H-bonding there making tertiary amines more basic than the primary amines. Due to delocalization of lone pair of electrons of the nitrogen atom on the benzene ring, aniline is a weaker base than ammonia. The basic strength of the substituted anilines, however, depends upon the nature of the substituent. Whereas electron-donating groups tend to increase, electron-withdrawing groups tend to decrease the basic strength. The base strengthening effect of the electron-withdrawing groups and base weakening effect of the electron-withdrawing group is, however, more pronounced at p-than at m-position. However, due to ortho-effect, o-substituted anilines regardless of the nature of substituent whether electron-donating or electron-withdrawing.   

Arrange the following amines in the decreasing order of their basic strength.

I > IV > III > II

Chlorine atom has both -I-effect and  +r-effect. Since -I-effect outweighs the +R-effect, therefore, all the three o-, m-, and p-chloroanilines  are weaker bases than aniline. However due to ortho-effect, o-chloroaniline is the weakest base. 

Further, since in p-chloroaniline both +R and -I-effect operate together but in m-chloroaniline only the -I-effect operates, therefore, p-chloroaniline is a stronger base than m-chloroaniline .