Law of Constant Proportions

The Law of Constant Proportion, also known as the Law of Definite Proportions, states that a chemical compound always contains the same elements combined in a fixed proportion of mass, regardless of its source or the method by which it is prepared. This means that the ratio of the masses of the constituent elements in a compound is always constant.

Example:

In water(H₂O)

• water always contains hydrogen and oxygen in the mass ratio of 1:8.
• whether it is obtained from a river, rainwater, or prepared in a laboratory.

• According to the law of constant composition, chemical compounds are made up of elements combined in a fixed ratio by mass.
• The elements may differ in physical state or appearance before combining, but once the compound is formed, its composition is uniform and fixed.
• This law applies only to pure substances (compounds) and not mixtures, because in mixtures, the components can be present in any proportion.

Examples:

1. Water (H₂O)

Atomic mass:

• Hydrogen = 1
• Oxygen = 16

Mass of hydrogen in water = 2 × 1 = 2
Mass of oxygen in water = 16

Mass Ratio = 2 : 16
Simplified ratio = 1 : 8

2. Carbon Dioxide (CO₂)

Atomic mass:

• Carbon = 12
• Oxygen = 16

Mass of carbon = 12
Mass of oxygen = 2 × 16 = 32

Mass Ratio = 12 : 32
Simplified ratio = 3 : 8

Limitations of the Law

The Law of Constant Proportions states that a chemical compound always contains the same elements in a fixed ratio based on mass. However, there are some exceptions where this ratio may vary slightly. These exceptions occur mainly due to the presence of isotopes and non-stoichiometric compounds, which can cause small changes in the mass ratio of elements in a compound.

1. Presence of Isotopes:

• Isotopes are atoms of the same element with the same atomic number but different mass numbers.
• This is due to a difference in number of neutrons

Example: Hydrogen has three isotopes

• Protium (¹H) – mass= 1
• Deuterium (²H) – mass = 2
• Tritium (³H) – mass =3

2. Non-Stoichiometric Compounds:

• In these compounds, elements do not combine in exact whole-number ratios.
• Their composition may vary slightly, especially in solid compounds like metal oxides.

Example: Iron Oxide (Fe_xO)

• The ratio of elements is not perfectly fixed under all condition.

3. Impure Samples:

• The Law of Constant Proportion applies only to pure compounds.
• If a substance contains impurities or is mixed, the observed mass ratio may change

Related Articles:

• Law of Conservation of Mass
• Chemical Formula