A given chemical compound always contains its component elements in fixed ratio and does not depend on its source and method of preparation.
Stoichiometry
Field: Stoichiometry
Sequence of Expressions
If two elements form more than one compound between them, then the ratios of the masses of the second element which combine with a fixed mass of the first element will be ratios of small whole numbers.
If two different elements combine separately with the same weight of a third element, the ratio of the masses in which they do so are either the same or a simple multiple of the mass ratio in which they combine.