For first order reaction k=t2.303log10(a−x)a
where, a = Initial concentration x = change in concentration during time ‘t’.
If 75% of the reaction was completed in 32 min,then k=322.303log10(100−75)100 =322.303log104 k=0.0433min−1
Hence, time required for the completion of 50% reaction t=0.04332.303log10(100−50)100 =0.04332.303log102 =16min