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Law of Definite Proportion

Law of Definite Proportion

Also known as Proust Law or the principle of constant composition, the law of definite proportions states that, "a given compound always contains exactly the same proportion of elements by mass."

In other words, a pure compound, the elements combine in definite proportions to each other.

Ex. IN water    H2O  the ratio proportion of hydrogen to oxygen is always 1:8.   That is 2 grams of hydrogen to every 16 grams of oxygen.


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