Dalton and Constant Composition

Introduction

Pure water from a mountain spring, from melted Antarctic ice or from a chemistry laboratory is exactly the same substance. If you break any sample of pure water down into its elements, you always find about 1 g of hydrogen for every 8 g of oxygen. This remarkable regularity is called the law of constant composition . Dalton's atomic theory gave the first convincing explanation of why it must be true.

Core explanation

The law. The law of constant composition (also called the law of definite proportions) states that a pure compound always contains the same elements combined in the same proportions by mass, however it is made and wherever it comes from.

Where it came from. Around 1799 the French chemist Joseph Proust studied copper carbonate. He compared samples found naturally in the ground with samples he made in the laboratory. Every pure sample contained copper, carbon and oxygen in the same proportions by mass. Proust did similar work with tin oxides and iron sulfides. Another chemist, Claude Berthollet, argued that compositions could vary smoothly, and the two men debated for years. Careful analysis eventually supported Proust.

Dalton's explanation. Dalton proposed that:

- all atoms of one element have the same mass; - a compound forms when atoms of different elements join in a fixed, simple whole-number ratio.

Put these together and constant composition follows. Suppose every particle of a compound contains one atom of A and one atom of B. If every A atom has the same mass, and every B atom has the same mass, then every particle has the same mass ratio of A to B. A large sample is just a huge number of identical particles, so the whole sample has that same ratio too. Doubling the size of the sample doubles the mass of each element, but the ratio does not change.

Some typical mass ratios.

Compound Elements Mass ratio --- --- --- Water, H₂O hydrogen : oxygen about 1 : 8 Carbon dioxide, CO₂ carbon : oxygen 3 : 8 Magnesium oxide, MgO magnesium : oxygen 3 : 2 Sodium chloride, NaCl sodium : chlorine about 23 : 35.5

Compounds versus mixtures. The law applies to compounds, not mixtures. Salt water can contain a little salt or a lot; air can be damp or dry. A mixture has a variable composition, while a pure compound has a fixed one. This is one of the clearest tests for telling the two apart.

Formulae

mass of element in sample = (mass ratio fraction of that element) × mass of sample

For example, in magnesium oxide the fraction of magnesium is 3 ÷ (3 + 2) = 3/5.

Step-by-step reasoning

To predict the mass of an element in a sample of a compound:

1. Write down the fixed mass ratio of the elements. 2. Add the parts of the ratio to find the total number of parts. 3. Work out the fraction of the total that is the element you want. 4. Multiply that fraction by the mass of the sample.

Visual explanation

Imagine a box full of identical water molecules, each drawn as one large red oxygen ball with two small white hydrogen balls attached. Scoop out a spoonful or a bucketful: each scoop contains only this same unit, repeated. Because the repeating unit never changes, the proportion of red to white mass is the same in every scoop.

Real-world analogy

A bicycle factory always builds each bicycle from one frame and two wheels. Whether it makes ten bicycles or ten thousand, the total mass of wheels compared with the total mass of frames stays the same, because the ratio in each bicycle is fixed.

Real-world example

Food and drug laboratories rely on constant composition every day. A tablet of pure paracetamol always contains carbon, hydrogen, nitrogen and oxygen in the same proportions by mass. Quality-control chemists analyse samples and compare the results with the expected values; a different composition warns that the tablet is impure or is not what the label says.

Why?

Why can a compound not have a "little extra oxygen" in some samples? Because each particle of the compound is built from a fixed number of each kind of atom. Adding more oxygen atoms would make a different particle, and therefore a different compound with different properties, not a slightly altered version of the same one.

Common misconception

"If you use more oxygen when burning magnesium, you get magnesium oxide with more oxygen in it." No. Extra oxygen is simply left over, unreacted. The magnesium oxide formed always has the 3 : 2 mass ratio of magnesium to oxygen.

Worked example

Question: 2.4 g of magnesium burns completely to form 4.0 g of magnesium oxide. What mass of magnesium oxide forms from 6.0 g of magnesium, and how much oxygen combines with it?

Reasoning: In the first experiment, oxygen = 4.0 − 2.4 = 1.6 g, so magnesium : oxygen = 2.4 : 1.6 = 3 : 2. The ratio is fixed, so 6.0 g of magnesium combines with 6.0 × 2/3 = 4.0 g of oxygen. The oxide formed has mass 6.0 + 4.0 = 10.0 g.

Answer: 10.0 g of magnesium oxide, containing 4.0 g of oxygen.

Quick check

1. Pure carbon dioxide from a car exhaust and from a fizzy drink are analysed. What will the carbon : oxygen mass ratio be in each? Answer: 3 : 8 in both, because a pure compound always has the same composition.

Exam focus

Be ready to state the law in words and to explain it with Dalton's ideas: fixed atom ratios plus fixed atom masses give fixed mass ratios. Calculation questions often give the masses from one experiment and ask you to scale them; always find the ratio first.

Advanced insight

A few solids, such as some metal oxides and sulfides, show small variations in composition because their crystals contain missing or extra atoms. Iron(II) oxide, for instance, usually has slightly fewer iron atoms than oxygen atoms. These are called non-stoichiometric compounds. Mass ratios can also shift very slightly because elements are mixtures of isotopes. For ordinary molecular compounds, however, the law holds extremely well.

Summary

The law of constant composition says a pure compound always contains the same elements in the same proportions by mass. Proust established it experimentally, and Dalton explained it: each particle of a compound contains atoms in a fixed whole-number ratio, and all atoms of an element have the same mass, so every sample has the same mass ratio. Mixtures, by contrast, have variable composition.

Practice questions

1. State the law of constant composition. Answer: A pure compound always contains the same elements combined in the same proportions by mass, regardless of its source or method of preparation. 2. Explain, using Dalton's ideas, why water always has the same hydrogen : oxygen mass ratio. Answer: Every water particle contains hydrogen and oxygen atoms in the same fixed ratio, and all hydrogen atoms have the same mass as each other, as do all oxygen atoms, so every sample has the same mass ratio. 3. 3.0 g of carbon combines with 8.0 g of oxygen to form carbon dioxide. What mass of oxygen combines with 9.0 g of carbon? Answer: The ratio is 3 : 8, so 9.0 g of carbon needs 9.0 × 8/3 = 24 g of oxygen. 4. Why does the law of constant composition not apply to sea water? Answer: Sea water is a mixture, and mixtures can contain their components in any proportions.