The Law of Reciprocal Proportions (also called the Law of Equivalent Proportions or Richter's Law) is one of the fundamental laws of chemical combination formulated by Jeremias Benjamin Richter in 1792.
The five laws of chemical combination are: 1) Conservation of mass, 2) Definite proportions, 3) Multiple proportions, 4) Reciprocal proportions, 5) Gay-Lussac's law of combining volumes.
Statement: 'If two different elements A and B each combine with a fixed mass of a third element C, then the ratio in which A and B combine with each other will be the same, or a simple multiple of, the ratio in which they combine with C.'
Classic Example: Consider hydrogen (H), oxygen (O), and sulphur (S):
Now, O and S combine (using H as the common element): ratio expected = 1:8 × 1:16 = 1:2 (simplified). In SO₂, ratio S:O = 1:1, and in SO₃ = 1:1.5 = 2:3 — these are simple multiples of the expected ratio. ✓
This law, along with the Laws of Definite Proportions and Multiple Proportions, was explained perfectly by Dalton's Atomic Theory. The law shows a fundamental mathematical harmony in how elements combine.
The Law of Reciprocal Proportions was proposed by German chemist **Jeremias Benjamin Richter** in 1792. It is therefore sometimes called Richter's Law.
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