Question

The rate constant of a first order reaction is $0.0693$ $mi{n}^{-1}$.

Time (in minutes) required for reducing an initial concentration of $20mol$ $li{t}^{-1}$ to $2.5mol$ $li{t}^{-1}$ is :

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

Answer 1

The correct option is B $30$

For a $1^{st}$ order reaction,
$t=\frac{2.303}{k}log\left[\frac{R_o}{R}\right]$
$t=\frac{2.303}{0.0693}log\left[\frac{20}{2.5}\right]$
t $=30.01min$

Option B is correct.