Page 137 - Chemistry ICSE Class X

Mole Concept and Stoichiometry 123

How is the relationship between molecular mass and
vapour density derived from Avogadro’s law
6JG XCRQWT FGPUKV[ 8 & QH C ICU KU FGſPGF CU VJG TCVKQ DGVYGGP VJG OCUU
of a certain volume of gas, to the mass of the same volume of hydrogen gas
under similar condition of temperature and pressure. Thus,

Mass of V mL of the substance in gaseous state
Vapour density of a substance =
Mass of V mL of hydrogen under similar conditions
According to the Avogadro’s law equal volumes of gases under similar
conditions should contain equal number of molecules. So, if V mL of any
gas contain n molecules, then
Mass of n molecules of the gaseous substance
V.D. of a substance =
Mass of n molecules of hydrogen
Mass of 1 molecule of the gaseous substance
or V.D. of a substance =
Mass of 1 molecule of hydrogen
Since, one molecule of hydrogen contains 2 atoms, hence
Mass of 1 molecule of the gaseous substance
V.D. =
2 × Mass of 1 atom of hydrogen
#U RGT FGſPKVKQP
Molecular mass of Mass of 1 molecule of the gaseous substance
the substance = Mass of 1 atom of hydrogen

Hence,

Molecular mass of the substance = 2 × V.D. of the substance
In general, therefore,
Molecular mass = 2 × Vapour density

How to determine the molecular formula of a gaseous
compound
The Avogadro’s law can also be used for determining the molecular formula
of a gaseous compound, if the volumes of the reactants and products
are known. This is illustrated by taking example of the reaction between
nitrogen and hydrogen gases to form ammonia. Experimentally, it is found
that one volume of nitrogen reacts with three volumes of hydrogen to form
two volumes of ammonia. Then, one can write,
Nitrogen + Hydrogen o Ammonia
1 vol 3 vol 2 vol
n molecules 3n molecules 2n molecules (Avogadro’s law)
1 1
molecule molecule 1 molecule
2 2
or 1 atom 3 atom 1 molecule (atomicity of nitrogen
and hydrogen = 2)
Thus, one molecule of ammonia contains one atom of nitrogen and
three atoms of hydrogen. Therefore, the molecular formula of ammonia is
NH .
3
Some Examples Based on Avogadro’s Law

EXAMPLE 5.10. 100 mL of a gas ‘A’ contains x molecules. How many molecules
of the gas ‘B’ will be present in 50 mL of ‘B’ under same conditions of temperature
and pressure?