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.

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. 

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.

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

The total pressure of a gaseous mixture equals the sum of the partial pressures of all individual gases.

Rate of diffusion of a gas is inversely proportional to the square root of its molar mass.

\[\frac{r_1}{r_2}=\sqrt{\frac{M_2}{M_1}}\]

\[\text{Rate of diffusion}=\frac{\text{Volume of gas diffused}}{\text{Time required for diffusion}}\]

"When gases combine or are produced in a chemical reaction, they do so in a simple ratio by volume, provided all gases are at the same temperature and pressure."

• Proposed by Gay-Lussac in 1808.
• e.g. 100 mL H₂ + 50 mL O₂ → 100 mL H₂O vapour (ratio = 2 : 1 : 2).
• The volume ratio of gaseous reactants to products agrees with their molar ratio.
• Volume of a gas is directly proportional to the number of moles (not inversely).

Statement: The volume remaining constant, the pressure of a given mass of gas increases or decreases by 1/273.15 of its pressure at 0°C for each 1°C rise or fall in temperature.

where β = pressure expansion coefficient = 1/273 per °C.

"Equal volumes of all gases at the same temperature and pressure contain equal numbers of molecules."

• Proposed by Avogadro in 1811.
• 1 mole of any gas at STP = 22.4 L (at 0°C, 1 atm) or 22.71 L (at 0°C, 1 bar — new IUPAC STP).
• 1 mole of any substance = 6.022 × 10²³ particles.

Avogadro's Law (Volume–Moles Relationship):

At constant temperature (T) and pressure (P), volume is directly proportional to number of moles.

• 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.