Law of constant proportions

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(Note: In the legend of the picture the colors for hydrogen and oxygen are swapped - hydrogen should be blue, oxygen red)
Clarification of the law of constant proportions based on the mass ratios of various molecules.

The law of constant proportions states that the elements in a certain chemical compound always occur in the same mass ratio . The mass ratio in sodium chloride, for example, is always 39% sodium to 61% chlorine .

This rule, established by Joseph-Louis Proust in 1797 and called Proust's law after him, is the starting point for the development of stoichiometry . John Dalton extended it to the law of multiple proportions .

These observations made by Proust and others were an essential step in Dalton's development of the atomic hypothesis . It is the easiest way to explain the constant mass ratio of the elements in sodium chloride: Sodium chloride is made up of the same number of sodium and chlorine particles . A chlorine particle is half the weight of a sodium particle. Other examples would be copper (I) sulfide {mass ratio m (Cu): m (S) about 4: 1, molar ratio n (Cu): n (S) = 2: 1; a copper particle is twice as massive as a sulfur particle) or sulfuric acid (with n (H): n (S): n (O) = 2: 1: 4, see figure and - in contrast to the difference in mass and quantity - below the order of magnitude (Amount of substance) }.

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