Law of Constant Composition Presentation

Topic 1 · Compound identity

Law of Constant Composition

A pure compound always contains the same elements in the same fixed mass ratio, no matter where the compound comes from or how much of it you have.

fixed mass ratio percent composition compound identity

Learning target

Use mass ratios and percent composition to decide whether a sample is a pure compound, a different compound, or a mixture.

H:O = 1:8

For pure water by mass.

Why it matters

Fixed composition helps chemists identify substances.

If two samples are the same pure compound, their elemental mass percentages must match. This idea supports chemical analysis, quality control, forensic testing, and formula determination.

Unknown identification

Elemental analysis can reveal whether an unknown sample matches a known compound.

Purity testing

A sample whose composition changes from place to place is not behaving like a pure compound.

Formula reasoning

Fixed composition connects measured mass data to empirical and molecular formulas.

Core concept

A compound has a fixed recipe by mass.

The law of constant composition is also called the law of definite proportions. It says that the elemental composition of a pure compound is constant.

Same elements

Every pure sample of water contains hydrogen and oxygen.

Same mass ratio

The masses follow the same ratio: 1 g H for every 8 g O.

Same compound

Changing the fixed ratio changes the chemical identity or creates a mixture.

Vocabulary

Mass ratio and percent composition describe the same fixed recipe.

A formula gives particle ratios. Because atoms have different masses, the formula also produces a fixed mass ratio and fixed mass percent.

Term	Meaning	Typical unit	Example
Mass ratio	Mass of one element compared with mass of another element in a compound	g:g or simplified ratio	Water has H:O = 1:8 by mass.
Percent composition	Percentage of total compound mass contributed by each element	% by mass	Water is about 11.19% H and 88.81% O.
Empirical formula	Simplest whole-number atom ratio in a compound	formula	CH₂O for glucose.
Molecular formula	Actual number of atoms in one molecule	formula	C₆H₁₂O₆ for glucose.
Mixture	Physical combination with variable composition	varies	Salt water can be dilute or concentrated.

Main relationship

Percent composition converts mass data into chemical identity evidence.

If a sample is pure, the mass percent of each element should match the compound’s fixed composition.

\[ \text{mass percent of element} = \frac{\text{mass of element}}{\text{mass of compound}} \times 100\% \]

Interpretation: The numerator is the mass of one element in the compound. The denominator is the total mass of the compound sample.

Fixed ratio test

If two pure samples have the same mass ratio, they may represent the same compound.

Variable ratio warning

If the ratio changes from sample to sample, the material is likely a mixture or a different compound.

Interactive simulation

Adjust the masses and test whether the sample matches water.

Pure water has a hydrogen-to-oxygen mass ratio of 1:8. Move the sliders to see whether the sample keeps the fixed composition.

Composition tester

Hydrogen mass: 2.0 g

Oxygen mass: 16.0 g

Calculated composition

H:O = 1:8.00

11.11% H and 88.89% O by mass

Matches pure water closely

Static fallback model

For pure water, 2.0 g of hydrogen combines with 16.0 g of oxygen. The ratio simplifies to 1:8.

H

O

The bar shows relative mass contribution, not particle size.

Dynamic relationship

A fixed compound follows a straight mass-ratio line.

For any pure compound, the mass of one element is proportional to the mass of the other element. Different compounds have different slopes because their fixed ratios are different.

The plotted point comes from the slider masses on the previous slide. A point on the line matches the selected compound’s fixed ratio.

Worked example

Does a sample match pure magnesium oxide?

A sample contains 12.15 g Mg and 8.00 g O. Pure MgO has one Mg atom for every one O atom. Using approximate atomic masses, Mg is 24.3 g/mol and O is 16.0 g/mol.

1

Find the expected mass ratio for MgO

For MgO, Mg:O by mass is \(24.3:16.0\), or about \(1.52:1\).

2

Find the sample mass ratio

\(\frac{12.15\ \text{g Mg}}{8.00\ \text{g O}}=1.52\)

3

Compare and conclude

The sample ratio matches the fixed ratio for MgO, so the data are consistent with pure magnesium oxide.

Final answer: The composition is consistent with MgO because the magnesium-to-oxygen mass ratio is about \(1.52:1\).

Common misconception

Same elements do not always mean the same compound.

Carbon monoxide and carbon dioxide both contain carbon and oxygen, but they are not the same compound because their fixed mass ratios are different.

Incorrect reasoning

“Both samples contain carbon and oxygen, so they must be the same compound.”

This ignores the fixed mass ratio.

Correct reasoning

CO and CO₂ contain the same elements, but CO has less oxygen per carbon than CO₂. Different fixed ratios mean different compounds.

Practice check

Use percent composition to test a sample.

Question: A compound sample has 4.00 g hydrogen and 32.00 g oxygen. Does it match the fixed composition of water?

Show answer

1

Find the mass ratio

H:O = \(4.00:32.00\)

2

Simplify

Divide both values by 4.00: H:O = \(1:8\)

3

Conclude

Yes. The sample matches water because water has a hydrogen-to-oxygen mass ratio of \(1:8\).

Reasonableness check

If the sample were pure water, oxygen should contribute most of the mass. Here oxygen is 32.00 g out of 36.00 g, which is 88.89% of the sample.

Apply the topic

Use fixed composition as a chemical identity test.

In later problems, you can use mass percent and fixed ratios to distinguish compounds from mixtures, verify formulas, and interpret experimental composition data.

Open the calculator

Practice mass-ratio and percent-composition calculations with guided checks.

Try related questions

Test whether you can identify fixed composition from formulas and data.

Formula

Gives a fixed atom ratio.

Mass ratio

Comes from atom ratios and atomic masses.

Identity

Fixed composition supports compound identification.

Final summary

Pure compounds have constant composition.

Fixed recipe

A pure compound always has the same elements in the same mass ratio.

Percent composition

Mass percent measures how much of the compound’s mass comes from each element.

Compounds versus mixtures

Compounds have fixed composition; mixtures can vary in composition.

Identity evidence

Changing the elemental ratio changes the compound or indicates a mixture.

Key idea: The law of constant composition links formulas, mass ratios, and percent composition to the identity of a pure chemical compound.