Question

In a first order reaction, the concentration of the reactant, decreases from 0.8 mol dm⁻³ to 0.4 mol dm⁻³ in 15 minutes. The time taken for the concentration to change from 0.1 mol dm⁻³ to 0.025 mol dm⁻³ is ____________.

Options

• 60 minutes

• 15 minutes

• 7.5 minutes

• 30 minutes

Answer 1

In a first order reaction, the concentration of the reactant, decreases from 0.8 mol dm⁻³ to 0.4 mol dm⁻³ in 15 minutes. The time taken for the concentration to change from 0.1 mol dm⁻³ to 0.025 mol dm⁻³ is 30 minutes.

Explanation:

Half life period `"t"_(1//2)` = 15 min.

Calculating the number of `"t"_(1//2)` required to decrease the concentration from 0.1 mol dm⁻³ to 0.025 mol dm⁻³,

\[\ce{0.1 ->[t_{{1/2}}] 0.05 ->[t_{{1/2}}] 0.025}\]

number of `"t"_(1//2)` = 2

∴ time taken = 2 × 15 = 30 min