Question

The rate constant (k) of a first order reaction is found out to be $0.0693{min}^{-1}$. A reactant having initial concentration of $20mol{L}^{-1}$ is reduced to $2.5mol{L}^{-1}$ in

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

Answer 1

The correct option is C $30min$

For a first order reaction,
$t=\frac{2.303}{k}\log \frac{[A{]}_0}{\left[A\right]}$

where $[A{]}_0$= initial concentration of reactant
$\left[A\right]$= present concertration of reactant
$k=\text{rate constant}=0.0693{min}^{-1}$
So, time required for concentration to change from $20mol{L}^{-1}$ to $2.5mol{L}^{-1}$

$t=\frac{2.303}{0.0693}\log \left[\frac{20}{2.5}\right]$
$t\approx 30min$
Hence, (c) option is correct.