Page 138 - Chemistry ICSE Class X

124 ICSE Chemistry – 10
Solution: A : B
Volume: 100 mL 50 mL
No. of molecules: x ?
According to the Avogadro’s law, equal volumes of two gases under similar
conditions would contain the same number of molecules. Therefore,
100 mL of gas B would contain x molecules
Hence,
x
50 mL of gas B would contain = x × 50 molecules = molecules
100 2
EXAMPLE 5.11. Samples of the gases, oxygen, nitrogen, carbon dioxide and
carbon monoxide under the same conditions of temperature and pressure
contain the same number of molecules, (say X). The molecules of oxygen (O )
2
occupy V litres and have a mass of 8 g.
(a) What is the volume occupied by

(i) X molecules of nitrogen (N )?
2
(ii) 2X molecules of carbon monoxide (CO)?
(b) What is the mass of CO in grams?
2
(c) In answering the above questions, which law would you use?
(Atomic masses are : C = 12, N = 14, O = 16)

Solution: The given data are rearranged as follows:
Gas: Oxygen Nitrogen Carbon Carbon
dioxide monoxide
No. of molecules: X X X X
Volume: V L
Mass of gas: 8 g

Equal volumes of gases under similar conditions of temperature and
pressure contain the same number of molecules. Therefore,
(a) (i) Volume occupied by X molecules of N = V L
2
(ii) Volume occupied by 2X molecules of CO = 2 × V L = 2V L
(b) On molar basis O { CO 2
2
1 mol 1 mol
2 × 16 g = 32 g (12 g + 2 × 16 g) = 44 g
Therefore, 32 g of O { 44 g of CO 2
2
44 g
So, 8 g of O = 32 g × 8 g of CO = 11 g
2
2
(c) In answering the above question, the Avogadro’s law has been
used.
EXAMPLE 5.12. The volumes of gases A, B, C and D are in the ratio of 1 : 2 : 2 : 4
under the same conditions of temperature and pressure.
(a) Which sample of gas contains the maximum number of molecules?
(b) If the temperature and pressure of gas A are kept constant, then
what will happen to the volume of A, when the number of its
molecules is doubled?
(c) If the ratio of gas volumes refers to the reactants and products of
a reaction, which gas law is being observed?
(d) If the volume of A is actually 5.6 L at STP, calculate the number of
molecules in the actual volume of D at STP. [Avogadro’s number:
23
6.02 × 10 ]