Question

The dissociation of nitrogen pentaoxide is a first order reaction. In first 24 min, 75% of nitrogen pentaoxide 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

The correct option is C Approximately 3%

$t_{75\%}=2t_{\frac{1}{2}}$
$\therefore t_{\frac{1}{2}}=\frac{24min}{2}=12min$
n = Number of half lives in 1 hr(60 min) $\frac{60}{12}=5$
Amount of substance left after 5 half lives
$={\left(\frac{1}{2}\right)}^5=\frac{1}{35}$
$\therefore \%ofamountleft=\frac{100}{32}=3.125\%\approx 3\%$