Question

In a first order reaction, the concentration of reactant, decrease from 0.8 M to 0.4 M in 15 minutes. The time taken for concentration to change from 0.1 M to 0.025 M is:

• A. 7.5 minutes
• B. 15 minutes
• C. 30 minutes
• D. 60 minutes

Answer 1

The correct option is C 30 minutes

Its a 1st order reaction,

$k=\frac{2.303}{t}log\frac{\left[A\right]}{[A-x]}$

So,

$k=\frac{2.303}{15}log\frac{\left[0.8\right]}{\left[0.4\right]}$

In the 2nd Case,

$k=\frac{2.303}{t^1}log\frac{\left[0.1\right]}{\left[0.025\right]}$

On substituting the value of k, We get

$t=30min$