Law of reciprocal proportion

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## Law of reciprocal proportion or Law of Equivalent proportion –

Richter gave this law.According to this law,

#### ” The weights of the two or more substances which separately react with same weight of a third substance are also the weights of these substances which react with each other or in a simple multiple of them. “

Ex – Hydrogen combine with sulphur and oxygen to form compounds H2S and H2O respectively.

In H2S                   2 parts hydrogen               32 parts sulphur

In H2O                   2 parts hydrogen               16 parts oxygen

According to the law of reciprocal proportion sulphur should combine with oxygen in the ratio of 32 : 16

( 2 : 1) or a simple multiple of it.

#### In SO2,

sulphur and oxygen combine in the ratio of 32 : 32 or 1 : 1.

The two ratios are related to each other as 2/1 :1/1 = 2:1

This is a simple multiple ratio.

#### In SO3,

sulphur and oxygen combine in the ratio of 32 : 48 or 2 : 3

### Solution )

#### In SO2 ,

mass of oxygen = 50 gm

mass of sulphur  = 100 – 50 = 50 gm

#### In H2O,

mass of hydrogen = 11.11 gm

mass of oxygen = 100 – 11.11 = 88.89  gm

Let us fix the mass of oxygen to 1 gm.

#### In H2O ,

88.89 gm oxygen combine with hydrogen = 11.11

1 gm oxygen combine with hydrogen = 11.11 /88.89 = 0.125 gm

#### In SO2,

50 gm of oxygen combine with sulphur = 50

1  gm of oxygen combine with sulphur = 50 / 50 = 1.0 gm

The ratio of masses of Hydrogen and sulphur which combine with the fixed mass of oxygen ( 1.0 gm) is ,

0.125 : 1.0 or 1 : 8

#### In H2S,

mass of sulphur = 94.11 gm

mass of hydrogen = 100 = 94.11 = 5.89

The ratio of masses of Hydrogen and sulphur  in H2S  is ,

5.89 : 94.11 = 1 : 16

Now comparing the ratio, 1/8 : 1/16= 2 : 1