There are $6.02\times {10}^{22}$ molecules each of ${\mathrm{N}}_2,{\mathrm{O}}_2$ and ${\mathrm{H}}_2$ which are mixed together at $760 \mathrm{mm}$ and $273 \mathrm{K}$. The mass of the mixture in grams is

$6.02\times {10}^{22}$ molecules of each ${\mathrm{N}}_2,{\mathrm{O}}_2$ and ${\mathrm{H}}_2$

$=\frac{6.02\times {10}^{22}}{6.02\times {10}^{23}}$ moles of each

$\text{ Weight of mixture }=\text{ weight of }0.1\text{ mole }{\mathrm{N}}_2+\text{ weight }$

of $0.1\text{ mole }{\mathrm{H}}_2+\text{ weight of }0.1\text{ mole of }{\mathrm{O}}_2$

$=(28\times 0.1)+(2\times 0.1)+(32\times 0.1)=6.2\mathrm{gm}$