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  4. A gas mixture contains 25 % He and 75 % CH4 by volume at a given temperature and pressure. The percentage by mass of methane in the mixture is approximately

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Q. A gas mixture contains $25\% \,He$ and $75\%\, CH_4$ by volume at a given temperature and pressure. The percentage by mass of methane in the mixture is approximately____

KCETKCET 2020Some Basic Concepts of Chemistry

Solution:

Since temperature and pressure are constant, we can assume it to be in STP condition.

Now, let the total volume be $100\, L$

Moles of $H e$ present $=\frac{25}{22.4}=1.12\, mol$

Mass of $H e$ present $=1.12 \times 4=4.48 \,g$

Moles of $C H_{4}$ present $=\frac{75}{22.4}=3.35\, mol$

Mass of $C H_{4}$ present $=3.35 \times 16=53.60 \,g$

Hence, the mass percentage of $C H_{4}=\frac{53.60}{58.08} \times 100=92.28 \%$