Short Answer

The Law of Multiple Proportion 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 element are in simple whole-number ratios. This means elements combine in definite patterns, not in random amounts.

For example, carbon and oxygen form two compounds—carbon monoxide (CO) and carbon dioxide (CO₂). If the amount of carbon is kept the same, the mass of oxygen in CO and CO₂ will be in a simple whole-number ratio of 1:2. This law supports Dalton’s atomic theory.

Detailed Explanation :

Law of Multiple Proportion

The Law of Multiple Proportion is an important law in chemistry that explains how elements combine to form different compounds. It was proposed by John Dalton in the early 19th century as part of his atomic theory. This law shows that chemical combinations follow simple and predictable rules based on whole numbers. It helped establish the idea that elements are made up of atoms that combine in fixed ratios.

The law applies to pairs of elements that can form more than one compound. It explains why these compounds differ in composition and properties, even though they are made from the same elements. This law forms a strong foundation for understanding chemical formulas, stoichiometry, and atomic structure.

Statement of the Law

The Law of Multiple Proportion states:

“If two elements combine to form more than one compound, the masses of one element that combine with a fixed mass of the other element are in simple whole-number ratios.”

This means:

Elements combine in simple numerical patterns.
Compounds formed from the same elements differ in fixed, whole-number proportions.
Ratios like 1:2, 2:3, 1:3, etc., are common.
Chemical combinations are not random.

Understanding the Law with Examples

The law becomes clearer with real examples from chemistry.

Example 1: Carbon and Oxygen

Carbon combines with oxygen to form two compounds:

Carbon monoxide (CO)
Carbon dioxide (CO₂)

If we keep the mass of carbon constant:

In CO, carbon combines with 16 g of oxygen.
In CO₂, carbon combines with 32 g of oxygen.

Ratio of oxygen masses = 16 : 32 = 1 : 2
This is a simple whole-number ratio, proving the law.

Example 2: Nitrogen and Oxygen

Nitrogen and oxygen form several compounds such as:

Nitrogen monoxide (NO)
Nitrogen dioxide (NO₂)
Nitrous oxide (N₂O)

If the mass of nitrogen is kept constant:

In NO, nitrogen combines with 16 g oxygen
In NO₂, nitrogen combines with 32 g oxygen
In N₂O, nitrogen combines with 8 g oxygen

The ratio of oxygen masses = 8 : 16 : 32 = 1 : 2 : 4

This simple ratio again supports the law.

Example 3: Sulphur and Oxygen

Sulphur forms multiple oxides:

Sulphur dioxide (SO₂)
Sulphur trioxide (SO₃)

Keeping the mass of sulphur the same:

In SO₂, sulphur combines with 32 g of oxygen
In SO₃, sulphur combines with 48 g of oxygen

Ratio = 32 : 48 = 2 : 3

Another simple whole-number ratio.

Why the Law is True

The law is true because:

Atoms combine in whole-number ratios
Atoms cannot be divided during chemical reactions
Compounds form by fixed numbers of atoms
Atoms of different elements have fixed masses

For example:

CO has 1 atom of carbon + 1 atom of oxygen
CO₂ has 1 atom of carbon + 2 atoms of oxygen

The increase from 1 oxygen atom to 2 oxygen atoms creates a simple ratio.

This law strongly supports Dalton’s theory that elements are made of atoms.

Importance of the Law of Multiple Proportion

The law plays a vital role in chemistry because it:

Helps determine chemical formulas
Supports the concept of atoms and molecules
Proves that elements combine in definite patterns
Helps distinguish between different compounds of the same elements
Forms the basis for stoichiometric calculations
Supports the Law of Definite Proportion and Conservation of Mass

It is also important in chemical analysis, laboratory experiments, and developing the atomic model.

Relationship with Modern Chemistry

In modern atomic theory:

Atoms combine in whole-number ratios because they cannot exist in fractional forms
The ratios depend on the number of atoms that join to form molecules
Molecular formulas like CO, CO₂, NO, NO₂ prove the law at the atomic level

Thus, modern chemistry strongly supports the Law of Multiple Proportion.

Conclusion

The Law of Multiple Proportion states that when two elements combine to form more than one compound, the masses of one element that unite with a fixed mass of the other element are in simple whole-number ratios. This law explains why different compounds of the same elements exist and supports the idea that atoms combine in definite ratios. It is an essential concept in understanding chemical formulas, atomic theory, and the behaviour of matter.