Arrange the following acids in increasing order of their acidic strength. I. II. III. IV.

Nitro group from para-position increases acidic strength by strong electron withdrawing resonance effect while from meta position by -I-effect only. Hence, I is the strongest acid followed by II. Between III and IV III is stronger acid due to -I-effect of chlorine only but in IY chlorine from para-position donates electron by resonance and withdraw electron by induction, hence weakest acid.