3. Atoms & Molecules
Molecular mass/Formula mass
Molecular mass expresses as to how many times a molecule of a substance is heavier than
th of the mass of an atom of carbon (carbon-12). Thus,
Molecular mass =
Molecular mass = 2 × vapour density Example
A molecule of water is 18 times heavier than
th of the mass of carbon atom. Therefore, the molecular mass of water is 18 u. Calculation of molecular mass from atomic masses
The molecules are made up of two or more atoms of different elements. Therefore, the molecular mass may be calculated as the sum of the atomic masses of all the atoms in a molecule of that substance. Example.
Ammonia has the formula, NH3
. It consists of one atom of N and three atoms of H. The atomic mass of N and H are 14.0 and 1 respectively. Therefore, the molecular mass of NH3
Molecular mass of NH3
= At. mass of N + 3 × At. mass of H
= 14 + 3 × 1 = 17 u Example.
Sulphuric acid has the formula H2
. It consists of two H, one S and four O atoms. The atomic masses of H, S and O are 1,32 and 16 respectively. Therefore, the molecular mass of H2
Molecular mass of H2
(2 × at. m. of H) + (1 × at. m. of S)
+ (4 × at. m. of O)
= (2 × 1) + (1 × 32) + (4 × 16) = 98 u Gram molecular mass
The molecular mass of a substance expressed in gram is called its gram molecular mass. Example
. Molecular mass of oxygen, O2
= 32 u
So, gram molecular mass of oxygen, O2
= 32 grams.
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