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 112th 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
 
=2×Massofhydrogen+1×Massofsulphur+4×Massofoxygen=2×1+1×32+4×16=98
 
I.e., one molecule of \(H_2SO_4\) is \(98\) times as heavy as 112th 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
 
=2×Massofhydrogen+2×Massofoxygen=2×1+1×16=18
 
I.e., one molecule of \(H_2O\) is \(18\) times as heavy as 112th of the mass of a carbon –\(12\).