The Law of Definite Proportions: Definition & Examples

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  • 0:01 Law of Definite…
  • 0:52 Examples
  • 2:30 Sample Problems
  • 5:00 Lesson Summary
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Lesson Transcript
Instructor: Nissa Garcia

Nissa has a masters degree in chemistry and has taught high school science and college level chemistry.

Have you ever had to double a baking recipe? The trick is to always keep the ingredient proportions the same as in the original recipe. This cooking principle follows the Law of Definite Proportions in chemistry, which is the focus of this lesson.

The Law of Definite Proportions

Have you ever wanted to make more food than a recipe called for? Let's say you have a chocolate cake recipe that feeds four people, but you're going to a birthday party that will have eight guests. You will need to double the recipe, which also means you'll need to double the amount of eggs, sugar, flour, butter, milk and chocolate for the recipe. The amount of each ingredient will double, but the proportions should stay the same. We can think about this principle when defining The Law of Definite Proportions, which states that any chemical compound will always contain a fixed ratio of elements by mass.

A French scientist named Joseph Proust conducted experiments that demonstrated this law, from the late 1700's to the early 1800's. The Law of Definite Proportions is also sometimes called Proust's Law.

Examples of the Law of Definite Proportions

Let us take, for example, the compound, water. Whatever the source of water, its composition is that of two atoms of hydrogen and one atom of oxygen.

Law of Definite Proportions: % by Weight of Hydrogen and Oxygen in Water

This figure shows that water, from any source, is always made up of two atoms of hydrogen and one atom of oxygen. If we calculate the molecular weight of water, we come up with 18 g/mol. In 1 mole of water, there are 2 grams of hydrogen and 16 grams of oxygen. By weight, we have a percentage of 11% hydrogen and 89% oxygen in one mole of water. This translates into a ratio of 1:8 of hydrogen to oxygen in water.

The Law of Definite Proportions illustrates that whatever the amount of water, whether it be 2 moles or 54 grams, the ratio of the amount of hydrogen to oxygen by weight will always be the same, just like the egg:sugar:butter ratio in chocolate cake should always be the same to maintain consistent taste.

Let's look at another compound made up of three atoms this time, such as ethyl alcohol, which consists of 2 atoms of Carbon, 6 atoms of Hydrogen and one atom of Oxygen.

Law of Definite Proportions in Ethyl Alcohol

According to the figure, we can calculate the percentage of each element in ethyl alcohol by weight, and we come up with 52% carbon, 13% hydrogen and 35% oxygen by weight. Based on the Law of Definite Proportions, the percentages of carbon, hydrogen and oxygen by weight in ethyl alcohol will always be the same, no matter what the amount.

Sample Problems

Let's look at some problems involving the Law of Definite Proportions in chemistry.

Problem 1

Sodium chloride, NaCl, contains 39% Na (sodium) and 61% Cl (chloride) by mass. If you have a 150-g sample of NaCl, how many grams of sodium and chloride are there in the sample?

Solution for Problem 1

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