Explanation

(a) When electric spark is passed, hydrogen reads with oxygen to form water $\left( H_{2}O \right).$ Each gram of hydrogen reacts with eight grams of oxygen. Thus $96\ gm$ of oxygen will be totally consumed together with $12\ gm$ of hydrogen. The gas left in the vessel will be $2\ gm$ of hydrogen $i.e.$
Number of moles $\mu = \frac{2}{2} = 1$
Using $PV = \mu RT \Rightarrow P \propto \mu \Rightarrow \frac{P_{2}}{P_{1}} = \frac{\mu_{2}}{\mu_{1}}$
($\mu_{1} =$ Initial number of moles $= 7 + 3 = 10$ and $\mu_{2} =$ Final number of moles $= 1$)
$\Rightarrow \frac{P_{2}}{1} = \frac{1}{10} \Rightarrow P_{2} = 0.1\ atm$