Question

Rate constant of a first order reaction is $0.0693\,\,{{\min }^{-1}}$ . If we start with $20\,mol\,{{L}^{-1}},$ it is reduced to $2.5\,mol\,{{L}^{-1}}$ in:

• A. 10 min
• B. 20 min
• C. 30 min
• D. 40 min

Answer 1

Answer: (C)

30 min

Answer 2

The correct answer is C:30min
For, 1st order reaction, $k=\frac{2.303}{t}=\log \frac{a}{(a-x)}$
Hence, $t=\frac{2.303}{0.0693}\times \log \frac{20}{2.5}$
$=\frac{2.303}{0.0693}\times \log \,{{(2)}^{3}}$
$=\frac{2.303}{0.0693}\times 3\times 0.3010=30\min$