Law of Definite Proportions — Revision Notes

⚡ 30-Second Revision

Law of Definite Proportions (Proust's Law) — A pure chemical compound always contains its elements in a fixed ratio by mass, regardless of source or preparation.
Key Idea — Constant composition for a specific compound.
Formula for Percentage Composition — $$
Distinction — Applies to *one* compound, unlike Law of Multiple Proportions (multiple compounds from same elements).
Example — Water ($H_2O$) always has H:O mass ratio of $1:8$.

2-Minute Revision

The Law of Definite Proportions, also known as Proust's Law or the Law of Constant Composition, is a fundamental principle stating that any pure chemical compound will always contain its constituent elements combined in the same fixed ratio by mass.

This ratio is constant, irrespective of where the compound came from or how it was made. For instance, whether water is from a river or synthesized in a lab, it will always have hydrogen and oxygen in a 1:8 mass ratio.

This law is a direct consequence of atoms combining in specific whole-number ratios to form molecules. It is crucial for distinguishing compounds from mixtures, as mixtures can have variable compositions.

For NEET, be prepared to define the law, differentiate it from other laws of chemical combination (especially Law of Multiple Proportions), and perform calculations involving percentage composition or mass ratios of elements within a compound.

Remember that non-stoichiometric compounds are rare exceptions to this rule.

5-Minute Revision

The Law of Definite Proportions, a cornerstone of quantitative chemistry, asserts that a pure chemical compound always possesses a constant elemental composition by mass. This means that for a specific compound, say $XY_2$, the mass ratio of element X to element Y will be the same in every pure sample, regardless of its origin or synthetic route. This principle, established by Joseph Proust, underpins our understanding of chemical identity.

Key Aspects to Remember:

1
Fixed Mass Ratio — The defining characteristic. For example, in $H_2O$, the mass ratio of H:O is always $2:16$ or $1:8$. In $CO_2$, the C:O mass ratio is always $12:32$ or $3:8$.
2
Purity — The law applies strictly to *pure* chemical compounds. Mixtures, by definition, have variable compositions.
3
Atomic Basis — This law is explained by Dalton's atomic theory: atoms combine in fixed whole-number ratios, and since atoms have fixed masses, the overall mass ratio of elements in a compound must also be fixed.

Calculations:

Percentage Composition — To find the percentage by mass of an element in a compound, use:

$$ *Example*: Calculate %O in $H_2O$. Molar mass of $H_2O = 2(1) + 16 = 18,\text{g/mol}$. Mass of O in $H_2O = 16,\text{g/mol}$. $$.

Finding Unknown Mass — If you know the mass ratio and the mass of one element, you can find the mass of the other.

*Example*: If 10g of a compound contains 2g of element A and 8g of element B, the A:B mass ratio is 1:4. If you have 20g of the same compound, it will contain 4g of A and 16g of B.

Distinction from Law of Multiple Proportions: The Law of Definite Proportions deals with the constant composition of a *single* compound. The Law of Multiple Proportions deals with the simple whole-number ratios of masses of one element that combine with a fixed mass of another element to form *two or more different* compounds. Be careful not to confuse these two.

Exceptions: Non-stoichiometric compounds (e.g., $Fe_{0.95}O$) are rare exceptions where the elemental ratio can vary slightly due to crystal defects. For NEET, assume the law holds unless explicitly stated otherwise or a non-stoichiometric compound is mentioned.

Prelims Revision Notes

Law of Definite Proportions (Proust's Law)

1. Definition: A pure chemical compound always contains its constituent elements in the same fixed ratio by mass, irrespective of its source or method of preparation.

2. Key Principle: Constant composition. For any given compound, the percentage by mass of each element is always the same.

3. Example:

* Water ($H_2O$): Always contains Hydrogen and Oxygen in a mass ratio of $1:8$. (Atomic mass H=1, O=16. So, $2 \times 1 : 1 \times 16 = 2:16 = 1:8$). * Carbon Dioxide ($CO_2$): Always contains Carbon and Oxygen in a mass ratio of $3:8$. (Atomic mass C=12, O=16. So, $1 \times 12 : 2 \times 16 = 12:32 = 3:8$).

4. Significance:

* Distinguishes compounds from mixtures (mixtures have variable composition). * Forms the basis for stoichiometry and quantitative analysis in chemistry.

5. Calculations:

* Percentage by Mass of an Element: $$ *Example:* For $NH_3$ (N=14, H=1): Molar mass $NH_3 = 14 + 3(1) = 17,\text{g/mol}$. $$. $$. * Finding Unknown Mass: If a compound has elements A and B in a mass ratio $x:y$, then for any sample of that compound, $Mass_A / Mass_B = x / y$.

6. Comparison with other Laws:

* Law of Conservation of Mass: Deals with total mass in a chemical reaction (mass is conserved). * Law of Multiple Proportions: Applies when two elements form *more than one* compound. States that the masses of one element combining with a fixed mass of the other are in simple whole-number ratios.

* Key Distinction: Law of Definite Proportions focuses on the constant composition of a *single* compound. Law of Multiple Proportions compares the compositions of *multiple* compounds formed from the same elements.

7. Exceptions (Advanced/Rare for NEET): Non-stoichiometric compounds (e.g., $Fe_{0.95}O$) do not strictly follow this law due to crystal defects, leading to slight variations in elemental ratios.

Vyyuha Quick Recall

Pure Compounds Fix Mass Ratios: Proust's Law states that Compounds have Fixed Mass Ratios of their elements.