Law of Constant Proportions: Definition & Examples

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  • 0:00 Law of Constant Proportions
  • 0:46 Elements in Compounds
  • 1:30 Proportions of Compounds
  • 3:28 Lesson Summary
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Lesson Transcript
Instructor: Laura Foist

Laura has a Masters of Science in Food Science and Human Nutrition and has taught college Science.

In this lesson we will learn what the law of constant proportions is and what is meant by the law. We will also look at examples of the law of constant proportions.

Law of Constant Proportions

A molecule of water will always have two hydrogen atoms and one oxygen atom, meaning that in a bottle of pure water the hydrogen to oxygen atom ratio will always be 2:1. This seems very logical to us today. But 300 years ago, scientists didn't understand compounds this way. However, in 1794, Joseph Proust published the Law of Constant Proportions, which says that a chemical molecule will always contains the same elements in the same proportion.

There are two parts to the law of constant proportions: 1) there will always be the same elements that make up a compound, and 2) the mass of these compounds will always be in the same proportion.

Elements in Compounds

Pure water will always have hydrogen and oxygen atoms. It will never contain nitrogen, carbon, or phosphorus; it will only have hydrogen and oxygen atoms. Pure glucose will only contain carbon, hydrogen, and oxygen but no other elements. Pure ammonia will only contain nitrogen and hydrogen.

The key word here is 'pure.' Typically water from your tap (or in your bottle) will have other elements mixed in with it to add flavor. So, it isn't pure water. In this case, the compound that you are drinking will have hydrogen, oxygen, calcium, magnesium, sodium, and many other elements. So, it is not pure water. But when a compound is pure then it will always contain the same elements.

Proportions of Compounds

Looking at water, we know that it contains hydrogen and oxygen. The chemical formula for water is H 2 O, so there are 2 hydrogen atoms and 1 oxygen atom. This ratio will always be true in a bottle of pure water no matter where the water comes from or what form the water is in. Whether it is solid, liquid, or gas, it will always have a ratio of two hydrogen atoms to one oxygen atom.

Since the mass of an element is known and constant, the mass proportion is also constant for a compound. With water, the mass of hydrogen is 1 g/mol and the mass of oxygen is 16 g/mol. So, there are 2 g of hydrogen per 16 g of oxygen in a water molecule; so the mass ratio of hydrogen to oxygen is 2:16; or, when simplified, 1:8.

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