Page 135 - Chemistry ICSE Class X

Mole Concept and Stoichiometry 121

What is Avogadro’s law
Avogadro proposed his famous hypothesis, known as Avogadro’s
hypothesis. The hypothesis (now known as Avogadro’s law) states dŽ ĐŽŵƉĂƌĞ ƚŚĞ ďĞŚĂǀŝŽƵƌ ŽĨ ŐĂƐĞƐ͕ ŝƚ ŝƐ
that, “under similar conditions of temperature and pressure equal ŶĞĐĞƐƐĂƌLJ ƚŽ ĐŚŽŽƐĞ ĐĞƌƚĂŝŶ ĮdžĞĚ ǀĂůƵĞƐ ŽĨ
ƚĞŵƉĞƌĂƚƵƌĞ ĂŶĚ ƉƌĞƐƐƵƌĞ͘
volumes of all gases contain the same number of molecules.” ^ƵĐŚ ĮdžĞĚ ǀĂůƵĞƐ ŽĨ ƚĞŵƉĞƌĂƚƵƌĞ ĂŶĚ ƉƌĞƐƐƵƌĞ
For example, if equal volumes of different gases are held in ĂƌĞ ĐĂůůĞĚ normal temperature and pressure
separate containers under similar conditions of temperature and (NTP), and standard temperature and pressure
pressure, then each container will contain the same number of (STP).
molecules. The total mass of each gas in the container may differ ͻ Normal temperature and pressure
from each other. dŚĞ ĂĐĐĞƉƚĞĚ EdW ǀĂůƵĞƐ ĂƌĞ
EŽƌŵĂů ƚĞŵƉĞƌĂƚƵƌĞ с ϬΣ Žƌ Ϯϳϯ <
EŽƌŵĂů ƉƌĞƐƐƵƌĞ с ϭ Ăƚŵ с ϳϲ Đŵ ,Ő
Atomicity с ϳϲϬ ŵŵ ,Ő с ϳϲϬ ƚŽƌƌ
ͻ Standard temperature and pressure
What is meant by atomicity dŚĞ ĂĐĐĞƉƚĞĚ ^dW ǀĂůƵĞƐ ĂƌĞ
^ƚĂŶĚĂƌĚ ƚĞŵƉĞƌĂƚƵƌĞ с ϬΣ с Ϯϳϯ <
The number of atoms of all the elements present in a molecule of ^ƚĂŶĚĂƌĚ ƉƌĞƐƐƵƌĞ с ϭ ďĂƌ с ϭϬ WĂ
5
any substance is called its atomicity. ;ϭ Ăƚŵ с ϭ͘ϬϭϯϮϱ ďĂƌͿ
ƒ Monatomic molecules. The molecules which contain only dŚĞ EdW ĂŶĚ ^dW ĐŽŶĚŝƟŽŶƐ ĂƌĞ ŶŽƚ ƚŚĞ ƐĂŵĞ͘
one atom in their molecule are called monatomic molecules. dŚŝƐ ĚŝīĞƌĞŶĐĞ͖ ŚŽǁĞǀĞƌ͕ ŝƐ ƐŵĂůů͘ dŚĞƌĞĨŽƌĞ͕ ŝŶ
Examples. Noble gases viz, helium, neon, argon, etc. are ĐŽŵŵŽŶ ƵƐĞ ďŽƚŚ ƚŚĞ ƚĞƌŵƐ ŵĂLJ ďĞ ƵƐĞĚ ĂƐ ŝĨ
monatomic gases. Therefore, the atomicity of helium (He), ƚŚĞ ƐĂŵĞ͘
neon (Ne) and argon (Ar) is one.

ƒ Diatomic molecules. The molecules which contain two atoms in their
molecule are called diatomic molecules. Examples. The molecules
of gases like hydrogen, oxygen, nitrogen, chlorine, etc. contain two
atoms. Therefore, molecules of hydrogen, oxygen, nitrogen, chlorine
are diatomic molecules. Thus, the atomicity of hydrogen (H ), oxygen
2
(O ), nitrogen (N ) and chlorine (Cl ) is two.
2
2
2
ƒ Triatomic molecules. The molecules which contain three atoms in their
molecule are called triatomic molecules. Examples. The molecules
of water and carbon dioxide are triatomic molecules. Therefore, the
atomicity of water (H O) and carbon dioxide (CO ) is three.
2
2
Applications of the Avogadro’s Law

Avogadro’s law has been found very useful in chemistry. The usefulness
of the Avogadro’s law lies in the fact that it differentiates between an atom dŚĞ ĂƚŽŵŝĐŝƚLJ ŽĨ
and a molecule. Some important applications of the Avogadro’s law are ͻഩKnjŽŶĞ ŝƐ ϯ
ͻഩWŚŽƐƉŚŽƌƵƐ ;LJĞůůŽǁͿ ŝƐ ϰ
described below. ͻഩ^ƵůƉŚƵƌ ŝƐ ϴ

How does Avogadro’s law explain Gay-Lussac’s law of
combining volumes
Consider this reaction,
a A + b B o c C + d D
Let, a molecules of A react with b molecules of B, and there be n
molecules per unit volume of A. Under similar conditions of temperature
and pressure, B should also have n molecules per unit volume. Then,

Volume of A involved in the reaction = 1 × a units
n