Page 147 - Chemistry ICSE Class X

Mole Concept and Stoichiometry 133
So, Molecular mass of K Cr O = 78 u + 104 u + 112 u = 294 u
2
2
7
Thus, 294 units of K Cr O contain 112 units of oxygen. Therefore,
2
7
2
Mass of oxygen 112 u
Percentage of oxygen in K Cr O = × 100 = × 100 = 38.1
2
2
7
Mass of K Cr O 7 294 u
2
2
Thus, K Cr O contains 38.1% oxygen.
7
2
2
(c) Al (SO ) Al S O { 2 atoms of Al + 3 atoms of S + 12 atoms of O
4 3
12
2
2 3
2 × 27 u 3 × 32 u 12 × 16 u
54 u 96 u 192 u
So, Molecular mass of Al (SO ) = (54 + 96 + 192) u = 342 u
2
4 3
Thus, 342 u of Al (SO ) contain 192 u of oxygen (O). Therefore,
4 3
2
192 u
Percentage of oxygen in Al (SO ) = × 100 = 56.1
4 3
2
342 u
Thus, Al (SO ) contains 56.1% of oxygen.
2
4 3
EXAMPLE 5.19. How many grams of Cr are there in 85 g of Cr S ? Atomic
2 3
masses are: Cr = 52 u and S = 32 u.
Solution: Cr S { 2 atoms of Cr + 3 atoms of S
2 3
2 × 52 u 3 × 32 u
104 u 96 u
Molecular mass of Cr S = (104 + 96) u = 200 u
2 3
So, 200 u of Cr S contain = 104 u of Cr
2 3
or 200 g of Cr S contain = 104 g of Cr
2 3
Thus,
85 g of Cr S would contain = 104 g × 85 g = 44.2 g of Cr
2 3
200 g
Determination of Empirical and Molecular Formula
What is meant by empirical formula
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“Empirical formula (or stoichiometric formula) of a substance is the
simplest formula which gives the lowest whole-number ratio between
the number of atoms of different elements present in the substance.”
What is meant by molecular formula
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“A shorthand notion for the molecule of a substance in terms of
symbols and number of atoms of each element present in it is called its
molecular formula.”
Molecular and empirical formulae of any compound can be determined
from the given composition of that compound. This is illustrated below.
How to determine the formula of an unknown compound
The chemical formula of a compound can be determined from the
composition of the compound. The composition of a compound is commonly
expressed in terms of percentage of each element present in it.
To determine the formula of an unknown compound, follow the
following steps:
Step 1. Determine the percentage of each element present in the compound
from the mass of each element present in a certain known mass of the
compound.