Page 146 - Chemistry ICSE Class X

132 ICSE Chemistry – 10

How is the percentage composition of a compound
determined from its formula
The percentage composition of a compound means the mass percentage
of each element present in the compound. The mass percentage of each
element can be determined from the molecular or empirical formula of the
compound as follows.
Step 1. Write the molecular or empirical formula of the compound.
Step 2. Write the name, number of atoms, and the total mass of each element
present in one molecule (or formula) of the compound.
Step 3. Obtain the molecular mass or formula mass of the compound by adding
the masses of all the atoms present in it.
Step 4. Calculate the mass percentage of each element by using the relationship:
Total mass of the element in one molecule of the compound
Mass percentage of an element = × 100
Molecular mass of the compound
This method of calculating the percentage composition of a compound
from its molecular formula is illustrated through the following examples:

EXAMPLE 5.17. Calculate the mass percentage of each element present in
water (H O).
2
Solution: The molecular formula of water is H O.
2
A molecule of water is made up of two hydrogen atoms and one oxygen atom.
Thus, one can write,
Molecular formula of water: H O = 2 atoms of H + 1 atom of O
2
We know Atomic mass of H = 1 u
and Atomic mass of O = 16 u
Therefore, Mass of two H atoms = 2 × 1 = 2 u
Mass of one O atom = 1 × 16 = 16 u
and Molecular mass of water = 2 u + 16 u = 18 u
So,
2 u
Mass percentage of hydrogen (H) in water = × 100 = 11.11
18 u
16 u
and Mass percentage of oxygen (O) in water = × 100 = 88.89
18 u
Therefore, water (H O) contains 11.11% of hydrogen (H), and 88.89% of oxygen
2
(O).
EXAMPLE 5.18. Calculate the mass percentage of oxygen in (a) HgO
(b) K Cr O (c) Al (SO ) . The atomic masses (in u units) are:
7
2
2
2
4 3
Hg = 200.6, O = 16, K = 39, Cr = 52, Al = 27, S = 32
Solution: (a) HgO { 1 atom of Hg + 1 atom of O
So, Molecular mass of HgO = (200.6 u + 16 u) = 216.6 u
Therefore,
Percentage of O in HgO = Mass of oxygen × 100
Mass of HgO
16 u
= × 100 = 7.39
216.6 u
Thus, HgO contains 7.39% oxygen.
(b) K Cr O { 2 atoms of K + 2 atoms of Cr + 7 atoms of O
2
7
2
2 × 39 u 2 × 52 u 7 × 16 u
78 u 104 u 112 u