Question

The dissociation of nitrogen pentoxide is a first order reaction. In first 24 min, 75% of nitrogen pentaxide is dissociated. What amount of nitrogen pentaoxide will be left behind after one hour of the start of reaction ?

• A. Approximately 1%
• B. Approximately 2%
• C. Approximately 3%
• D. None

Answer 1

Answer: (C)

Approximately 3%

For first-order reaction,

$t_{75\%}=2t_{1/2}$
$\therefore t_{1/2}=\frac{24min}{2}=12min$
n = Number of half lives in 1 hr (60 min) = $\frac{60}{12}$ = 5
Amount of substance left after 5 half lives-
${\left(\frac{1}{2}\right)}^5=\frac{1}{32}$
$\therefore \%ofamountleft=\frac{100}{32}=3.125\%\approx 3\%$