Relative Molecular mass (RMM) and its calculation — lesson. Science State Board, Class 10.

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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

$\frac{1}{12^{th}}$ 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

$\begin{array}{l} = (2 \times Mass of hydrogen) + (1 \times Mass of sulphur) + (4 \times Mass of oxygen) \\ = (2 \times 1) + (1 \times 32) + (4 \times 16) \\ = 98 \end{array}$

I.e., one molecule of

$H_2SO_4$ is

$98$ times as heavy as

$\frac{1}{12^{th}}$ 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

$\begin{array}{l} = (2 \times Mass of hydrogen) + (2 \times Mass of oxygen) \\ = (2 \times 1) + (1 \times 16) \\ = 18 \end{array}$

I.e., one molecule of

$H_2O$ is

$18$ times as heavy as

$\frac{1}{12^{th}}$ of the mass of a carbon –

$12$ .

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6. Relative Molecular mass (RMM) and its calculation

Theory: