October 4, 2025

What Is The Law Of Definite Proportions

Q: Who is credited with discovering the Law of Definite Proportions?

The French chemist Joseph Proust is credited with this law based on his work in the late 18th century. For this reason, it is often called Proust's Law.

Q: How does this differ from the Law of Multiple Proportions?

The Law of Definite Proportions applies to a single compound. The Law of Multiple Proportions applies when two elements can form *more than one* compound, stating that the mass ratios of the second element combining with a fixed mass of the first will be small whole numbers.

Q: Does the Law of Definite Proportions apply to mixtures?

No, it does not. A mixture, such as salt dissolved in water, can have a variable composition. You can add more salt or more water, changing the ratio, but it remains a salt water mixture.

Q: Are there any exceptions to the Law of Definite Proportions?

While it is a foundational principle, there are exceptions known as non-stoichiometric compounds (or berthollides), which can have a variable composition within a small range. However, for the vast majority of compounds studied in introductory chemistry, the law holds true.

Learn about the Law of Definite Proportions (Proust's Law), a fundamental principle in chemistry stating that a compound always contains the same elements in the same proportion by mass.

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Defining the Law of Definite Proportions

The Law of Definite Proportions states that a given chemical compound, regardless of its source or method of preparation, always contains its component elements in a fixed ratio by mass. This means the composition of a pure substance is constant.

Section 2: Core Principle

This fundamental law of chemistry, also known as Proust's Law after its discoverer Joseph Proust, establishes the identity of a compound. It distinguishes compounds from mixtures, as mixtures can have variable proportions of their components. For a compound, the ratio of the masses of the elements within it does not change.

Section 3: A Practical Example with Water

Consider pure water (H₂O). It is always composed of two hydrogen atoms and one oxygen atom. By mass, water is approximately 11.19% hydrogen and 88.81% oxygen. Whether you analyze water from an ice cap, a river, or a laboratory synthesis, this mass ratio will remain exactly the same.

Section 4: Why This Law is Important

The Law of Definite Proportions was a crucial step in the development of modern chemistry and John Dalton's atomic theory. It provided evidence that elements combine in consistent, predictable ways, which supported the idea that matter is composed of discrete atoms that bond in simple, whole-number ratios.

Frequently Asked Questions

Who is credited with discovering the Law of Definite Proportions?

How does this differ from the Law of Multiple Proportions?

Does the Law of Definite Proportions apply to mixtures?

Are there any exceptions to the Law of Definite Proportions?