Rank the compounds in each group in order of increasing acidity.

a.

b.

The strength of an acid depends on the ease with which it can be deprotonated [i.e., the ease with which it can give a proton $\left(H^{+}\right)$ ].

The presence of an electron-withdrawing group in an acidic compound increases its acidity as it stabilizes the conjugate base (species generated by the abstraction of a proton from the acid) of the acid.

a.

Compounds in a

Among the given compounds, 1 and 3 consists of electron-withdrawing substituents.

Compound 1 is more acidic than 3 as chlorine is more electronegative than bromine.

The order of acidity of the given compounds is:1>3>2

b.

Compounds in b

The order of electron-withdrawing tendency of the substituents on the phenyl rings are in the order:

${\mathrm{NO}}_2>\mathrm{Cl}>{\mathrm{CH}}_3$

The acidity of the compound increases as the electron-withdrawing tendency of the substituent increases.

Therefore, the acidity of the given compounds are in the order:

$2>3>1$