### Theory:

Molecules have their own mass because they are made up of atoms. The $$C$$-$$12$$ scale is used to measure the mass of a molecule of an element or compound. Hence, it is called Relative Molecular Mass.
The relative molecular mass of a molecule is the ratio between the mass of one molecule of the substance to $\frac{1}{{12}^{\mathit{th}}}$ mass of an atom of carbon-$$12$$.
Relative molecular mass is only a ratio. Hence, it has no unit. If the molecular mass of a compound is expressed in grams, it is called Gram Molecular Mass.

Example:

Gram Molecular Mass of $$H_2O$$ $$=$$ $$18$$ $$g$$
Gram Molecular Mass of $$CO_2$$ $$=$$ $$44$$ $$g$$
Gram Molecular Mass of $$NH_3$$ $$=$$ $$17$$ $$g$$
Gram Molecular Mass of $$HCl$$ $$=$$ $$36.5$$ $$g$$

How to calculate the relative molecular mass of a molecule?

The relative molecular mass of a molecule is calculated by adding the relative atomic masses of all the atoms in the molecule.
Calculating relative molecular mass
Example: $$1$$

Calculate the relative molecular mass of sulphuric acid ($$H_2SO_4$$).

Solution:

Sulphuric acid contains,

Hydrogen ($$H$$) $$=2$$ atoms

Sulphur ($$S$$) $$=1$$ atoms

Oxygen ($$O$$) $$=4$$ atoms

Relative molecular mass of sulphuric acid

$\begin{array}{l}=\left(2×\mathit{Mass}\phantom{\rule{0.147em}{0ex}}\mathit{of}\phantom{\rule{0.147em}{0ex}}\mathit{hydrogen}\right)+\left(1×\mathit{Mass}\phantom{\rule{0.147em}{0ex}}\mathit{of}\phantom{\rule{0.147em}{0ex}}\mathit{sulphur}\right)+\left(4×\mathit{Mass}\phantom{\rule{0.147em}{0ex}}\mathit{of}\phantom{\rule{0.147em}{0ex}}\mathit{oxygen}\right)\\ =\left(2×1\right)+\left(1×32\right)+\left(4×16\right)\\ =98\end{array}$

I.e., one molecule of $$H_2SO_4$$ is $$98$$ times as heavy as $\frac{1}{{12}^{\mathit{th}}}$ of the mass of a carbon –$$12$$.
Example: $$2$$

Calculate the relative molecular mass of water ($$H_2O$$).

Solution:

Water contains,

Hydrogen ($$H$$) $$=2$$ atoms

Oxygen ($$O$$) $$=4$$ atoms

Relative molecular mass of water

$\begin{array}{l}=\left(2×\mathit{Mass}\phantom{\rule{0.147em}{0ex}}\mathit{of}\phantom{\rule{0.147em}{0ex}}\mathit{hydrogen}\right)+\left(2×\mathit{Mass}\phantom{\rule{0.147em}{0ex}}\mathit{of}\phantom{\rule{0.147em}{0ex}}\mathit{oxygen}\right)\\ =\left(2×1\right)+\left(1×16\right)\\ =18\end{array}$

I.e., one molecule of $$H_2O$$ is $$18$$ times as heavy as $\frac{1}{{12}^{\mathit{th}}}$ of the mass of a carbon –$$12$$.