Question

A first order reaction has a rate constant of ${10}^{-2}se{c}^{-1}$. How long will it take for a 20 g sample to become 5 g?

• A. 138.6

Answer 1

The correct option is A 138.6
k = ${10}^{-2}$ = $\frac{0.693}{t_{\frac{1}{2}}}$
$t_{\frac{1}{2}}$= $\frac{0.693}{{10}^{-2}}$

$t_{\frac{1}{2}}$= 69.3 seconds

It takes two half lives for 20 g to reduce to 5 g,

therefore, the time required = $2\times 69.3$
=138.6 seconds