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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 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 \(HCl\) \(=\) \(36.5\) \(g\)
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
I.e., one molecule of \(H_2SO_4\) is \(98\) times as heavy as 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
I.e., one molecule of \(H_2O\) is \(18\) times as heavy as of the mass of a carbon –\(12\).
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