Question

Both of the gas samples represented above are at the same temperature and pressure. The mass of $H_2$ in the one liter container is $0.20$ gram. The mass of $X$ in the two-liter container is $8$ grams. The molecular weight of $X$ is :

• A. $10$ g/mol
• B. $20$g/mol
• C. $40$g/mol
• D. $60$g/mol
• E. $80$ g/mol

Answer 1

Answer: (C)

$40$g/mol

The volume of a gas is directly proportional to its number of moles. The number of moles is the ratio of mass to molar mass.
$\frac{1}{2}=\frac{\frac{W_{H2}}{M_{H2}}}{\frac{W_X}{M_X}}$
$\frac{1}{2}=\frac{\frac{0.20}{2}}{\frac{8}{M_X}}$
$\frac{1}{2}=\frac{0.1}{8}\times M_X$
$M_X=\frac{1}{2}\times \frac{8}{0.1}$
$M_X=40$ g/mol
The molecular weight of $X$ is $40$ g/mol.