Question

The rate constant of a first order reaction is 6.9 × 10^–3s^–1. How much time will it take to reduce the initial concentration to its 1/8^th value?

Options

• 100 s

• 200 s

• 300 s

• 400 s

Answer 1

300 s

Explanation:

[A] = [A]₀ `(1/2)^n`

where [A]₀ = initial concentration

`1/8 = 1(1/2)^n ; n = 3`

`t^(1/2) = 0.693/(6.9 xx 10^(-3))` = 100 sec

∴ Time = `n xx t_(1/2)` = 3 × 100 = 300 sec.