Moles of oxygen, $n_{O_{2}}=\frac{4}{32}=0.125\, mol$
Moes of hydrogen, $n_{H_{2}}=\frac{2}{2}=1.00 \,mol$
Temperature $=273\, K$, volume $=1$ litre
$\Rightarrow$ Total pressure $=\frac{\left(n_{O_{2}}+n_{H_{2}}\right) \times R T}{V}$$=\frac{1.125 \times 0.0821 \times 273}{1}$$=25.215$ atm