Revision: Mole Concept and Stoichiometry >> Gay-Lussac's Law and Avogadro's Law Chemistry (English Medium) ICSE Class 10 CISCE

A temperature scale with absolute zero (zero kelvin) as the starting point is called the absolute scale or the kelvin scale.

The volume of a given mass of a dry gas varies inversely as the pressure and directly as the absolute temperature.

V ∝ \[\frac {1}{P}\] × T or \[\frac {PV}{T}\] = k (constant)

If volume changes from V₁​ to V_2​, pressure from P_1​ to P₂​, and temperature from T₁ to T₂​, then:

\[\frac {P_1V_1}{T_1}\] = \[\frac {P_2V_2}{T_2}\] = k (constant)

The reactant which is completely used up in a reaction is known as Limiting reagent or Limiting reactant.

A molecule is the smallest particle of an element or a compound that can exist by itself; it never breaks up except for taking part in a chemical reaction.

An atom is the smallest particle of an element that can take part in a chemical reaction; however, it may or may not exist independently. 

Avogadro s law states that "equal volumes of all gases under similar conditions of temperature and pressure contain the same number of molecules."

The number of atoms in a molecule of an element is called its atomicity. 

It states that the volume of a given mass of dry gas is inversely proportional to its pressure at a constant temperature.
P₁V₁ = P₂V₂ = k at constant temperature

It states that volume of a given mass of a dry gas is directly proportional to its absolute (kelvin) temperature, if the pressure is kept constant.

OR

The pressure remaining constant, the volume of a given mass of a dry gas increases or decreases by 1/273 of its volume for each 1°C increase or decrease in temperature respectively.

\[\frac {V_1}{T_1}\] = \[\frac {V_2}{T_2}\] = k at constant pressure

When gases react, they do so in volumes which bear a simple ratio to one another, and to the volume of the gaseous product, provided that all the volumes are measured at the same temperature and pressure.

• Types of Molecules: Monoatomic (He), Diatomic (H₂), Triatomic (O₃), Tetratomic (P₄), Octatomic (S₈).
• Homoatomic & Heteroatomic: Same atoms = homoatomic (O₃), different atoms = heteroatomic (HCl).
• Gay-Lussac’s Law: Gases react in simple volume ratios at the same temperature and pressure.
• Avogadro’s Law: Equal gas volumes at the same conditions have an equal number of molecules.
• Molecular Formula from Volumes: Volume ratios help find formulas, e.g., NH₃ from 2 NH₃ → 1 N₂ + 3.