Page 136 - Chemistry ICSE Class X

122 ICSE Chemistry – 10

and, Volume of B involved in the reaction = 1 × b units
n
Therefore, the ratio of volumes of the reacting gases A and B is,
a : b or a : b
n n
Since a and b are whole numbers (molecules cannot be in fractions),
hence the volumes of the reacting gases bear a simple whole number
ratio to each other. This is the Gay-Lussac’s law of combining volumes.

How is the atomicity of an elementary gas determined
using Avogadro’s law
To illustrate the determination of the atomicity of an elementary gas, let
us consider the reaction between hydrogen and chlorine to form hydrogen
chloride.
It has been observed experimentally that hydrogen and chlorine react
Amedeo Avogadro in equal volumes. Then, one can write,
;ϭϳϳϲͲϭϴϱϲͿ
ŵĞĚĞŽ ǀŽŐĂĚƌŽ ďŽƌŶ ŝŶ dƵƌŝŶ ;/ƚĂůLJͿ Atomicity of hydrogen and chlorine
ŽďƚĂŝŶĞĚ ĚĞŐƌĞĞƐ ŝŶ WŚŝůŽƐŽƉŚLJ Ăƚ ƚŚĞ Hydrogen + Chlorine o Hydrogen chloride
ĂŐĞ ŽĨ ϭϯ͕ ŝŶ :ƵƌŝƐƉƌƵĚĞŶĐĞ Ăƚ ϭϲ ĂŶĚ
ŝŶ ŚƵƌĐŚ ůĂǁ Ăƚ ϮϬ͘ /Ŷ ϭϴϬϵ͕ ǀŽŐĂĚƌŽ 1 vol 1 vol 2 vol
ďĞĐĂŵĞ WƌŽĨĞƐƐŽƌ ŽĨ WŚLJƐŝĐƐ Ăƚ ƚŚĞ n molecules n molecules 2n molecules (Avogadro’s law)
1
1
ZŽLJĂů ŽůůĞŐĞ Ăƚ sĞƌĐĞůůŝ͘ /Ŷ ϭϴϭϭ͕ ŚĞ 2 molecule 2 molecule 1 molecule
ƉŽƐƚƵůĂƚĞĚ ŚŝƐ ĨĂŵŽƵƐ ŚLJƉŽƚŚĞƐŝƐ Since one molecule of hydrogen chloride contains one atom of
ǁŚŝĐŚŝƐŶŽǁ ŬŶŽǁŶĂƐ ǀŽŐĂĚƌŽ͛Ɛ
ůĂǁ͘ /Ŷ ϭϴϮϬ͕ ǀŽŐĂĚƌŽ ǁĂƐ ĂƉƉŽŝŶƚĞĚ hydrogen and one atom of chlorine, hence,
ƉƌŽĨĞƐƐŽƌ ŽĨ DĂƚŚĞŵĂƟĐĂů WŚLJƐŝĐƐ Ăƚ 1 molecule of hydrogen = 1 atom of hydrogen
ƚŚĞhŶŝǀĞƌƐŝƚLJŽĨdƵƌŝŶ͘ 2
ǀŽŐĂĚƌŽ ŝŶ ϭϴϭϭ ƐƵŐŐĞƐƚĞĚ ƚŚĂƚ ƚŚĞ or, 1 molecule of hydrogen = 2 atoms of hydrogen
ĂƚŽŵŝĐ ĂŶĚ ŵŽůĞĐƵůĂƌ ŵĂƐƐĞƐ ĐĂŶ ďĞ Therefore, Atomicity of hydrogen = 2
ĞdžƉƌĞƐƐĞĚŽŶĂŶĂƚŽŵŝĐŵĂƐƐƐĐĂůĞ Similarly, 1 molecule of chlorine = 1 atom of chlorine
ďĂƐĞĚ ŽŶ ƚŚĞ ĐŽŵƉĂƌŝƐŽŶ ŽĨ ĂƚŽŵŝĐͬ 2
ŵŽůĞĐƵůĂƌ ŵĂƐƐĞƐ ǁŝƚŚ ƚŚĞ ŵĂƐƐ ŽĨ Ă or, 1 molecule of chlorine = 2 atoms of chlorine
ƌĞĨĞƌĞŶĐĞ ĂƚŽŵ͘ /Ŷ ƚŚŝƐ ŵĞƚŚŽĚ͕ ƚŚĞ Therefore, Atomicity of chlorine = 2
ĂƚŽŵŝĐ ŵĂƐƐĞƐ ĂƌĞ ĞdžƉƌĞƐƐĞĚ ƌĞůĂƟǀĞ
ƚŽ ƚŚĂƚ ŽĨ Ă ƐƚĂŶĚĂƌĚ ƌĞĨĞƌĞŶĐĞ ĂƚŽŵ͘ Thus, the atomicity of both hydrogen and chlorine is two.
ĂƌůŝĞƌ͕ ŚLJĚƌŽŐĞŶ ĂŶĚ ŽdžLJŐĞŶ ĂƚŽŵƐ
ǁĞƌĞ ƵƐĞĚ ĂƐ ƚŚĞ ƌĞĨĞƌĞŶĐĞ ĂƚŽŵƐ ĨŽƌ Atomicity of nitrogen and oxygen
ĚĞƐĐƌŝďŝŶŐ ĂƚŽŵŝĐ ŵĂƐƐĞƐ͘ /Ŷ ϭϵϱϳ͕ Similarly, we can determine the atomicity of nitrogen and oxygen.
KůĂŶĚĞƌ ĂŶĚ EŝĞƌ ƉƌŽƉŽƐĞĚ ĂŶ ĂƚŽŵŝĐ
ŵĂƐƐ ƐĐĂůĞ ďĂƐĞĚ ŽŶ ƚŚĞ ĐŚŽŝĐĞ ŽĨ Nitrogen + Oxygen o Nitric oxide
ϭϮ
ĐĂƌďŽŶͲϭϮ ŝƐŽƚŽƉĞ͕ ;ĚĞƐŝŐŶĂƚĞĚ ĂƐ ͕ 1 vol 1 vol 2 vol
ϲ
ϭϮ
ƐŝŵƉůLJ ĂƐ Ϳ͘ n molecules n molecules 2n molecules (Avogadro’s law)
1 molecule 1 molecule 1 molecule
2 2
1 atom 1 atom 1 molecule
Thus, 1 molecule of nitrogen = 1 atom of nitrogen
2
1 molecule of nitrogen = 2 atoms of nitrogen
Therefore, Atomicity of nitrogen = 2
Also, 1 molecule of oxygen = 1 atom of oxygen
2
1 molecule of oxygen = 2 atoms of oxygen

Therefore, Atomicity of oxygen =2