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 $25^{\large\circ}$

Question

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