The mass of Hydrogen at S.T.P. that is present in a vessel which can hold 4 grams of oxygen under similar conditions is

Equal volumes of different gas contain equal no. of molecule (or) moles. But not equal mass.

$32 \mathrm{grams} {\mathrm{O}}_2=2 \mathrm{grams} {\mathrm{H}}_2$

$4 \mathrm{grams} {\mathrm{O}}_2=\frac{2}{32}\times 4$

$=0.25 \mathrm{grams} {\mathrm{H}}_2 \mathrm{can} \mathrm{fit} \mathrm{in} \mathrm{vessel}$