Question

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

• A. 4 min
• B. 8 min
• C. 24 min
• D. 16 min

Answer 1

The correct option is D 16 min

Since it is a first order reaction, for 75% completion,
$k=\frac{2.303}{32}log\left(\frac{1}{0.25}\right)$
Now, let t is the time taken for $50\%$ completion, then
$k=\frac{2.303}{t}\left(\frac{1}{0.5}\right)$
So,
$\frac{2.303}{32}log4=\frac{2.303}{t}log2$
$\Rightarrow \frac{1}{16}=\frac{1}{t}$
$\Rightarrow t=16min$