Question

In the reaction, $A\to$ product, $\frac{-dA}{dt}=K\left[A\right].$ If we start with $10M$ of $A$, then after one natural life time, concentration of $A$ decreased to :

• A. $5M$
• B. $2.5M$
• C. $\frac{10}{e}M$
• D. $\frac{10}{e^2}M$

Answer 1

The correct option is C $\frac{10}{e}M$

For first order reaction,

$C=C_o{e}^{-Kt}$

At time $t=\frac{1}{K}$

$C=C_o\times e^{-K\times \frac{1}{K}}=\frac{10}{e}M$ as $C_o=10M$