Question

A first order reaction has a specific reaction rate of ${10}^{-2}{s}^{-1}$. How much time will take for 20 g of the reactant to reduce to 5 g?

• A. 238.6 seconds
• B. 138.6 seconds
• C. 346.5 seconds
• D. 693.0 seconds

Answer 1

The correct option is B 138.6 seconds

$t_{1/2}=\frac{0.693}{{10}^{-2}}$ seconds
For the reduction of 20 g of reactant to 5 g, two $t_{1/2}$ is required.
$\therefore t=2\times \frac{0.693}{{10}^{-2}}$ second
=138.6 seconds