Page 139 - Chemistry ICSE Class X

Mole Concept and Stoichiometry 125
(e) Using your answer in (d), state the mass of D, if the gas is dinitrogen
oxide (N O). (N = 14, O =16).
2
Solution: The given data are written as follows:
Gas: A B C D

Volume ratio: 1 : 2 : 2 : 4
(a) The number of molecules is proportional to the volume. Hence, gas D will
contain the maximum number of molecules.

(b) Since the number of molecules depends directly on the volume, the volume of
A will get doubled.
(c) The gas law being observed is Gay-Lussac’s law of combining volume for
gases.
(d) Volume of A at STP = 5.6 L
So, Volume of D at STP = 4 × 5.6 L = 22.4 L

22.4 L of any gas at STP will contain one Avogadro’s number of molecules. So,
No. of molecules in the actual volume of D = 6.02 × 10 23
(e) The mass of any gas occupying a volume of 22.4 at STP is equal to its
molar mass. Hence
Mass of gas D = Mass of 1 mole of N O
2
= (2 × 14 + 1 × 16) g = (28 + 16) g = 44 g
EXAMPLE 5.13. The gases chlorine, nitrogen, ammonia and sulphur dioxide
are collected under the same conditions of temperature and pressure. Copy
the following table which gives volume of gases collected and the number
of molecules x in 20 L of nitrogen. Complete the table giving the number of
molecules in the other gases in terms of x.

Gas Volume/L No. of molecules
Chlorine 10 —
Nitrogen 20 x
Ammonia 20 —

Sulphur dioxide 5 —
Solution: The number of molecules in any sample of a gas is proportional to its
volume. So,
10 L x
(a) No. of molecules in 10 L of chlorine = × x =
20 L 2
20 L
(b) No. of molecules in 20 L of ammonia = × x = x
20 L
5 L x
(c) No. of molecules in 5 L of sulphur dioxide = × x =
20 L 4
x x
So, the table can be completed by writing , x and in the blanks in order
from the top. 2 4