Question

$70\%$ of a first order reaction was completed in $70min$. What is the half-life of the reaction?

• A. $4.2min$
• B. $42min$
• C. $4.2hr$
• D. $42s$

Answer 1

The correct option is B $42min$

As this is a first order reaction we can use $k_1=\frac{2.303}{t}log\frac{a}{a-x}$ to get value of $k_1$.

So putting value is above equation we get $k_1=\frac{2.303}{70}log\frac{1}{0.3}$

Hence, $k_{!}=$0.017

Now to calculate half-life we can use $t_{1/2}=\frac{0.693}{k}$

Putting values in above equation we get

Half-life $=42$ min