Law of multiple proportions, Avogadro’s hypothesis

Proposed by John Dalton in 1803.
States that when two elements combine to form more than one compound, the masses of one element that combine with a fixed mass of the other are in the ratio of small whole numbers.
For example, carbon and oxygen form CO (carbon monoxide) and CO₂ (carbon dioxide). The mass ratio of oxygen combining with 12g of carbon is 16:32 or 1:2.
Demonstrates the concept of discrete atoms and fixed chemical composition in compounds.
Forms the basis of Dalton's atomic theory.

Highlights the existence of definite chemical formulas for compounds.
Strengthens the atomic theory of matter.
Helps in distinguishing between different chemical compounds of the same elements.

Proposed by Amedeo Avogadro in 1811.
States that equal volumes of gases, at the same temperature and pressure, contain an equal number of molecules.
Defines the relationship between the volume and the number of particles in a gas.
Explains the molecular composition of gases such as H₂, O₂, and Cl₂.
Gave rise to the concept of the mole, with 1 mole of a substance containing 6.022 × 10²³ particles (Avogadro's number).

Used in determining the molecular formula of compounds.
Forms the basis of the ideal gas law (PV = nRT).
Helps calculate the molar volume of gases, which is 22.4 L at STP for 1 mole of gas.
Supports the distinction between atoms and molecules in chemical reactions.

Essential for understanding stoichiometry in chemical reactions involving gases.
Explains the behavior of gases under varying conditions of temperature and pressure.
Forms a key concept in the study of physical chemistry.

The Law of Multiple Proportions may not be valid for non-stoichiometric compounds.
Avogadro's Hypothesis is accurate for ideal gases but has deviations for real gases at high pressure or low temperature.