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Law of Reciprocal Proportions

The law of reciprocal proportions was proposed by Jeremias Ritcher in 1792. It states that, "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."


Oxygen and sulfur react with copper to give copper oxide and copper sulfide, respectively. Sulfur and oxygen also react with each other to form SO2. Therefore,

in CuS:

Cu:S = 63.5:32

in CuO:

Cu:O = 63.5:16

S:O = 32:16

S:O = 2:1

Now in SO2:

S:O = 32:32

S:O = 1:1

Thus the ratio between the two ratios is the following:

\[\dfrac{2}{1} : \dfrac{1}{1} - 2:1\]

This is a simple multiple ratio.