Law of reciprocal proportions

The law that, when two elements combine separately with a fixed mass of a third, their combining masses relate in a simple ratio to the masses in which they combine with each other.

Equivalent weight: The mass of an element that combines with or replaces a fixed reference amount of another substance. Equivalent masses provide a numerical way to express the combining amounts used in reciprocal comparisons.
Jeremias Benjamin Richter: A German chemist who developed the study of chemical stoichiometry and introduced the term stoichiometry. Richter formulated reciprocal proportions while comparing the combining weights of acids and bases.
Gravimetric analysis: A quantitative analytical method that determines a substance's amount from the mass of a product of known composition. Reliable mass measurements are the experimental basis for testing reciprocal combining ratios.
Law of conservation of mass: In a closed system, the total mass remains constant during a chemical reaction. It concerns total reaction mass, whereas reciprocal proportions compare elemental combining masses.
Chemical formula: A symbolic notation that specifies the elements in a substance and their relative amounts. Formulas reveal the atomic proportions from which compound mass ratios can be calculated.
Stoichiometry: The quantitative study of relationships among substances in chemical reactions and compounds. The reciprocal law belongs to stoichiometry's early system of chemical combining weights.
Chemical stoichiometry: The calculation of quantitative relationships between reactants and products using chemical equations. Stoichiometric calculations apply fixed elemental ratios to compounds and reactions.
Law of definite proportions: A chemical compound contains its constituent elements in fixed proportions by mass. It fixes composition within one compound rather than comparing separate and direct combinations.
Combining capacity: The capacity of an element to combine with other elements, often represented by valence or oxidation state. Relative combining capacities help explain why reciprocal mass ratios reduce to simple proportions.
Chemical revolution: The eighteenth-century transformation of chemistry associated with new theories of combustion and quantitative methods. Its turn toward measurement made systematic laws of chemical combination possible.

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