Page 134 - Chemistry ICSE Class X

120 ICSE Chemistry – 10

EXAMPLE 5.7. 200 mL of CO is collected at STP when a mixture of acetylene
2
and oxygen is ignited. Calculate the volume of acetylene and oxygen at STP in
original mixture
2C H (g) + 5O (g) o 4CO (g) + 2H O(l)
2
2
2
2
2
Solution: The stoichiometry of the reaction gives
2C H (g) + 5O (g) o 4CO (g) + 2H O(l)
2
2
2
2
2
Volume ratio: 2 vol 5 vol 4 vol
Volume of CO : 200 mL
2
So,
Volume of C H (g) in the original mixture = 2 × 200 mL = 100 mL
2
2
4
Volume of oxygen in the original mixture = 5 × 200 mL = 250 mL
4
EXAMPLE 5.8. Ammonia is oxidised according to the reaction,
4NH (g) + 5O (g) o 4NO(g) + 6H O(l)
2
3
2
How many litres of NO are formed when 90 litres of oxygen react with ammonia
at STP?
Solution: The reaction stoichiometry gives,
4NH (g) + 5O (g) o 4NO(g) + 6H O(l)
2
2
3
Volume ratio: 4 vol 5 vol 4 vol
Volume of reactant: — 90 L
From the volume ratio,
Volume of NO formed = 4 vol × 90 L = 72 L
5 vol
EXAMPLE 5.9. 24 mL of marsh gas (CH ) was mixed with 106 mL of oxygen and
4
then exploded. On cooling, the volume of the mixture became 82 mL of which
58 mL was unchanged oxygen. Which law does this experiment support? Explain
with calculations.
Solution: The combustion of marsh gas (CH ) is described by the reaction,
4
CH (g) + 2O (g) o CO (g) + 2H O(l)
4
2
2
2
Volume ratio: 1 vol 2 vol 1 vol 0
Initial volume: 24 mL 106 mL — 0
Unreacted volume: — 58 mL —
Then, Volume of oxygen reacted = 106 mL – 58 mL = 48 mL
Volume of carbon dioxide formed = 82 mL – 58 mL = 24 mL
From the calculation, we are
V(CH ) : V(O ) : V (CO )
2
4
2
24 mL : 48 mL : 24 mL
1 : 2 : 1
The volumes of the gaseous reactants and products are in a simple ratio.
Thus, this experiment support the Gay-Lussac’s law of gaseous volumes.
Avogadro’s Law
An Italian physicist, Amedeo Avogadro in 1811 suggested that the smallest
particle in gases (except noble gases) were not atoms but molecules. These
molecules could contain two, three or more atoms.
For example, the number of atoms in one molecule of certain gases
are,
Helium (He) Oxygen (O ) Nitrogen (N ) Hydrogen (H )
2
2
2
1 2 2 2
Chlorine (Cl ) Ozone (O ) Sulphur dioxide (SO )
2
3
2
2 3 3