Page 33 - Chemistry ICSE Class IX

The Language of Chemistry 21
Molecular Mass and Relative Molecular Mass

What is the molecular mass of a substance
Average mass of a molecule of any substance is known as its molecular
mass. Molecular mass is designated as M. Since molecular mass is actual
mass, hence it has the unit of mass, XK\., gram, kilogram, atomic mass unit, dŚĞ ŵŽůĞĐƵůĂƌ ŵĂƐƐ ŽĨ Ă ƐƵďƐƚĂŶĐĞ
etc. The accepted standard unit of molecular mass is atomic mass unit (u). ŝƐ ĞƋƵĂů ƚŽ ƚŚĞ ƐƵŵ ŽĨ ĂƚŽŵŝĐ
Thus, the molecular mass of a substance may be de ned as follows: ŵĂƐƐĞƐ ŽĨ Ăůů ƚŚĞ ĂƚŽŵƐ ƉƌĞƐĞŶƚ ŝŶ Ă
“The average mass of a molecule of a substance expressed in atomic ŵŽůĞĐƵůĞ ŽĨ ƚŚĂƚ ƐƵďƐƚĂŶĐĞ͘
mass units is termed as its molecular mass.”

What is relative molecular mass of a substance
The relative molecular mass (M ) of any substance is de ned as follows:
T
“The relative molecular mass of a substance is the average mass of
12
its molecule as compared with the mass of a C atom taken as 12 units.”
6
Relative molecular mass of a substance,
Average mass of a molecule of the substance
M =
T
12
1 × (Mass of an atom of C)
12 6
The relative molecular mass (M ) is a pure number and has no unit.
T
How is the molecular mass of a substance calculated
Molecule are composed of two or more atoms of the same or different
elements. Mass is an additive property. So, the molecular mass of any
substance can be obtained by adding together the atomic masses of all
the atoms present in one molecule of the substance.
For example, the molecular formula of water is H O. Therefore,
2
Molecular mass of H O = (2 × Atomic mass of hydrogen) +
2
(1 × Atomic mass of oxygen)
We know that,
Atomic mass of hydrogen = 1 u /ƚ ŵĂLJ ďĞ ŶŽƚĞĚ ƚŚĂƚ ƚŚĞ ŵĂŐŶŝƚƵĚĞƐ
ŽĨ ƚŚĞ ƌĞůĂƟǀĞ ŵŽůĞĐƵůĂƌ ŵĂƐƐ ĂŶĚ
and Atomic mass of oxygen = 16 u
ŵŽůĞĐƵůĂƌ ŵĂƐƐ ĂƌĞ ĞƋƵĂů͘ dŚĞ ƚǁŽ
Therefore, ĚŝīĞƌ ŽŶůLJ ŝŶ ƚŚĞŝƌ ƵŶŝƚƐ͘
Molecular mass of H O = (2 × 1 u) + (1 × 16 u) = (2 u + 16 u) = 18 u
2

Composition of a Compound

A compound contains two or more elements combined in a certain xed
ratio. The composition of a compound can be described by the mass
percentage of each element present in it. This may be determined by any of
the methods described below.
How to determine the composition of a compound
Composition of a compound is commonly expressed as the mass percentage
of each element present in it. To determine the mass percentage of an element
in any compound, we should know the mass of that element present in a
certain known mass of the compound.
Let us suppose, 9 g of a compound contain Y g of an element A.
Then,
Mass percentage of the element A = Mass of element A × 100
Mass of the compound