Question

$t_{1 / 2}$ for a first order reaction is $10\, \min$. Starting with $10\, M$, the rate after $20\, \min$ is

• A. $0.0693\, M\ \min^{-1}$
• B. $0.0693\times 5\,M\, \min^{-1}$
• C. $0.0693\times 2.5\,M\,\min^{-1}$
• D. $0.0693\times 10\,M\, \min^{-1}$

Answer 1

Answer: (C)

$0.0693\times 2.5\,M\,\min^{-1}$

Answer 2

Given, $t_{1 / 2}=10\, \min$ $t=20\, \min$ $N_{0}=10\, M$ $\Rightarrow t=n t_{1 / 2}$ $20=n \times 10$ $\therefore n=2$ $\Rightarrow \frac{N}{N_{0}}=\left(\frac{1}{2}\right)^{n}$ $\Rightarrow \frac{N}{10}=\left(\frac{1}{2}\right)^{2}$ or $\frac{N}{10} =\frac{1}{4}$ $\therefore N =\frac{10}{4}=2.5\, M$ $\therefore$ Rate $=k[A]$ $=0.0693 \times 2.5\, M\, \min ^{-1}$