This law states that "The composition of a compound is always the same regardless of how the compound was made or obtained.
The law of constant composition is a chemistry law which states samples of a pure compound always contain the same elements in the same mass proportion. This law, together with the law of multiple proportions, is the basis for stoichiometry in chemistry.
In other words, no matter how a compound is obtained or prepared, it will always contain the same elements in the same mass proportion. For example, carbon dioxide (CO2) always contains carbon and oxygen in a 3:8 mass ratio. Water (H2O) always consists of hydrogen and oxygen in a 1:9 mass ratio.
Law of Constant Composition History
Discovery of this law is credited to the French chemist Joseph Proust. He conducted a series of experiments from 1798 to 1804 that led him to believe chemical compounds consisted of a specific composition. Keep in mind, at this time most scientists thought elements could combine in any proportion, plus Dalton's atomic theory was just beginning to explain each element consisted of one type of atom.
Law of Constant Composition Example
When you work with chemistry problems using this law, your goal is to look for the closest mass ratio between the elements. It's okay if the percentage is few hundredths off! If you're using experimental data, the variation might be even larger.
For example, let's say you want to demonstrate, using the law of constant composition that two samples of cupric oxide abide by the law. The first sample was 1.375 g cupric oxide, which was heated with hydrogen to yield 1.098 g of copper. For the second sample, 1.179 g of copper was dissolved in nitric acid to produce copper nitrate, which was then burned to produce 1.476 g of cupric oxide.
To work the problem, you need to find the mass percent of each element in each sample. It doesn't matter whether you choose to find the percent of copper or of oxygen. You would just subtract one value from 100 to get the percent of the other element.