Question

$60\%$ of a zero order reacton completes in 4 hours. $30\%$ of the same reaction completes in:

• A. $1hr$
• B. $2hr$
• C. $3hr$
• D. $4hr$

Answer 1

The correct option is B $2hr$

For a zero order reaction,
$\text{Rate constant,}K=\frac{{\left[A\right]}_0-\left[A\right]}{t}$

where,
${\left[A\right]}_0$= initial concentration of reactant
$\left[A\right]$ = concentration of reactant at time 't'
when 60% of a zero order reaction completes,
$\left[A\right]=\frac{40{\left[A\right]}_0}{100}$
$\text{Rate constant,}\left(K\right)=\frac{{\left[A\right]}_0-\frac{40}{100}[A{]}_0}{4}=0.15[A{]}_0$

When 30% of the reaction completes,
$\left[A\right]=\frac{70{\left[A\right]}_0}{100}$
$\text{Rate constant,}\left(K\right)=\frac{{\left[A\right]}_0-\frac{70}{100}[A{]}_0}{t}$
$t=\frac{{\left[A\right]}_0-\frac{70}{100}[A{]}_0}{K}$

$t=\frac{0.3[A{]}_0}{0.15[A{]}_0}$

$t=2hours$
$\therefore$ hence option b is correct