Question

The mass of $5.6{\mathrm{dm}}^3$ of a certain gas at STP is $12.0\mathrm{g}$.

Calculate the relative molecular mass of the gas.

Answer 1

The sum of all the individual atomic masses of all the atoms is the relative molecular mass/relative formula mass.

The sum of the relative atomic masses of the elements present in a formula unit is the relative formula mass of a substance.

M is the abbreviation for relative formula mass.
This mass is also known as the relative molecular mass if the substance is made up of simple molecules.

we know that $1{\mathrm{dm}}^3=1\mathrm{L}$

Given
$5.6{\mathrm{dm}}^3=5.6\mathrm{L}$
$5.6\mathrm{L}$ of gas has mass $12\mathrm{g}$
$22.4\mathrm{L}$ of gas has mass $\frac{12}{56}\times \frac{224}{10}\times 10$
$=48\mathrm{g}$

The relative molecular mass of the gas is $=48\mathrm{g}$.