$A \rightarrow$ Product ;

given $t_{1 / 2}=1\,hr$

for a zero order reaction

$t_{\text {completion}}=\frac{\left[A_{0}\right]}{k}=\frac{\text {initial} \,\operatorname{conc}}{rate\, {\text{on}}\,{\text{ stant}}}$

$\therefore $ $t_{1 / 2}=\frac{\left[A_{0}\right]}{2 K}$

or $ k=\frac{\left|A_{0}\right|}{2 t_{1 / 2}}=\frac{2}{2 \times 1}=1\,mol\,lit$  $^{-1}\, h r^{-1}$

Further for a zero order reaction

$k=\frac{d x}{d t}=\frac{\text { change in concentration }}{\text {time}}$

$I=\frac{0.50-0.25}{time}$

$\therefore $ time $=0.25\, h r$