Question

Which of the following is incorrect for the first-order reaction is?

• A. $t_{0.5}=100s,t_{0.75}=200s$
• B. $t_{0.75}=32$ min $t_{0.5}=16$min
• C. Both the above
• D. $t_{0.5}=100s,t_{0.75}=150s$

Answer 1

The correct option is D $t_{0.5}=100s,t_{0.75}=150s$

For first order reaction, $t_{1/2}=\frac{0.693}{k}$ which is

independent of concentration.

So, it takes same time to reduce to $\frac{1}{2}$ and to

reduce from $\frac{1}{2}$ to $\frac{1}{4}$.

So, $2t_{0.5}=t_{0.75}$

In option D, $t_{0.5}=1005$.

$t_{0.75}=1505$.

But $t_{0.75}$ must be 2005.

So, option D is incorrect.