The notion that the ratio by mass of compounds consumed in a chemical reaction is always the same.
When two pure substances react to form a compound, they do so in a definite proportion by mass. For example, when water is formed from the reaction between hydrogen and oxygen, the 'definite proportion' is 1 g of H for every 8 g of O.
in any chemical compound, the elements are always combined in the same proportion by mass.
a given compound always contains exactly the same proportion of elements by mass
(chemistry) law stating that every pure substance always contains the same elements combined in the same proportions by weight
Different samples of a pure compound always contain the same elements in the same proportions by mass.
One of the fundamental observations of modern chemistry, the law of definite proportions states that, in Compounds, the elements combine in proportion with each other, by mass.