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# 1 litre capacity flask containing $HN_3$ at 1 atm and $25^{\large\circ}C$. A spark is passed through untill all the $HN_3$ is decomposed into $N_2$ and $H_2$. Calculate the pressure of gases left at $25^{\large\circ}$

$(a)\;1\;atm\qquad(b)\;2\;atm\qquad(c)\;3\;atm\qquad(d)\;4\;atm$

$2NH_3 \rightarrow N_2 +3H_2$
at same P and T
Number of moles after reaction $n_2 = 2\times$ number of moles before reaction $(n_1)$
$\therefore P\propto n$
$\therefore \large\frac{P_1}{P_2} = \large\frac{n_1}{n_2}$
$\;\;\;\;\;\;\;\;\;\;=\large\frac{1}{2}$
Pressure after reaction = 2 atm