Question

75% of a first order reaction was completed in 32 minutes. When was 50% of the reaction completed?

(a) 24 minutes

(b) 16 minutes

(c) 8 minutes

(d) 4 minutes

Answer 1

Correct option is (b) 16 minutes

Fraction left after 32 minutes = \(\frac{100 -75}{100} = \frac 14 = (\frac 12)^2\)

Hence, 2 x t_1/2 = 32 min or t_1/2 = 16 min.