The law of definite proportions says that samples of a compound will always contain the same proportion of elements by mass. It is based on the principle that a compound is always made up of the same elements and that they all have constant weights.
Because of the law of definite proportions, a compound should always retain the same weight ratio between elements. An example of the law of definite proportions is that water will always be approximately one-ninth hydrogen by mass. It is also known as Proust's Law and was set forth in 1797 by Joseph Proust. He found two distinct iron oxides that had constant ratios, one with 27 percent oxygen by mass and one with 48, which led him to create the law.