Page 143 - Chemistry ICSE Class X

Mole Concept and Stoichiometry 129

What is relative molecular mass of a substance
The relative molecular mass (M QH CP[ UWDUVCPEG KU FGſPGF CU HQNNQYU
r
“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 /ƚ ŵĂLJ ďĞ ŶŽƚĞĚ ƚŚĂƚ ƚŚĞ ŵĂŐŶŝƚƵĚĞƐ
M = ŽĨ ƚŚĞ ƌĞůĂƟǀĞ ŵŽůĞĐƵůĂƌ ŵĂƐƐ ĂŶĚ
r
12
1 × (Mass of an atom of C)
12 6 ŵŽůĞĐƵůĂƌ ŵĂƐƐ ĂƌĞ ĞƋƵĂů͘ dŚĞ ƚǁŽ
ĚŝīĞƌ ŽŶůLJ ŝŶ ƚŚĞŝƌ ƵŶŝƚƐ͘
The relative molecular mass (M ) is a pure number and has no unit.
r
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 + (1 × Atomic mass of oxygen)
2
of hydrogen)
We know that,
Atomic mass of hydrogen = 1 u
and Atomic mass of oxygen = 16 u
Therefore,
Molecular mass of H O = (2 × 1 u) + (1 × 16 u) = (2 u + 16 u) = 18 u
2

Formula Mass

What is the formula mass of a substance
For ionic compounds, such as sodium chloride, there is no discrete individual
molecule. Instead, these are the aggregates represented by a formula of the dŚĞ ĐŽŶĐĞƉƚ ŽĨ ĨŽƌŵƵůĂ ŵĂƐƐ
–
+
+
–
type, (A B ) . Solid sodium chloride is represented by the formula (Na Cl ) , ŝƐ ĂƉƉůŝĐĂďůĞ ŽŶůLJ ƚŽ ƚŚĞ ŝŽŶŝĐ
n
n
or (NaCl) . Thus, the simplest formula for sodium chloride can be written as ƐƵďƐƚĂŶĐĞƐ͘
n
NaCl. So, the concept of formula mass is used for ionic compounds.
The sum of the atomic masses of all atoms in a formula unit of a
substance is called the formula mass of the substance.
How is the formula mass of an ionic substance calculated
The formula mass of any ionic compound is expressed as the sum of the
atomic masses of all the atoms present in its formula.
For example, for NaCl,
Formula mass of NaCl = (1 × Atomic mass of Na) + (1 × Atomic mass of Cl)
We know that
Atomic mass of Na = 23 u
and Atomic mass of Cl = 35.5 u
So, Formula mass of NaCl = (1 × 23 u) + (1 × 35.5 u)
= (23 + 35.5) u = 58.5 u
Thus, the molecular mass (or empirical formula mass) of a substance is equal
to the sum of the atomic masses of all the elements present in its molecular (or
empirical) formula, taking into account the number of atoms of each element present
in its molecular (or empirical) formula. For example,