Laws of Chemical Combination

When elements combine to form a compound, they do so according to the laws of chemical combination. They are:

1. The law of Conservation of mass

2. The law of Definite/Constant proportions

3. The law of Multiple proportions

4. The law of Reciprocal proportions

The Law of Conservation of Masses

It states that matter is neither created nor destroyed during a chemical reaction but changes from one form to another. It simply means that the total mass of the products of a chemical reaction is equal to the mass of the reacting substances of a chemical reaction.

Calculate the mass of silver chloride precipitate formed when 10.10g of silver trioxonitrate(V) reacts with 14.10g of sodium chloride to form 9.58g of sodium trioxonitrate(V) .

​2.

Sodium chloride solution reacts with silver trioxonitrate(V) to produce silver chloride and sodium trioxonitrate(V)

​1.

EXAMPLES:

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The Law of Constant Composition

It states that all pure samples of a particular chemical compound contain similar elements combined in the same proportion by mass.

                                           OR

All pure samples of the same compound have the same percentage composition by mass.

​Example:

Three samples of Copper(II) oxide were prepared thus:-

1. By adding concentrated trioxonitrate(V) acid to copper and heating the solution to dryness.

2. By heating strongly the mixture of copper(II) tetraoxosulphate(VI) solution and sodium hydroxide

3. By heating copper trioxocarbonate(IV) to dryness.

Analysis of the three samples shows that in:

1. 2.25g of Copper(II)oxide contains 2.01g of copper

2. 2.15g of copper(II) oxide contains 1.92g of copper

3. 2.44g of copper(II) oxide contains 2.18g of copper

Show that the results illustrates the law of multiple proportion.

​The Law of Multiple Proportion

It states that if two elements A and B combine together to form more than one chemical compound, then the several masses/various masses of one element A which combines chemically with a fixed mass of B is in a simple multiple ratio.

i.e:-

1. Iron and Oxygen form brown, Iron (III) oxide, Fe₂O₃ and black, Iron(II) oxide, FeO.

2. Lead and Sulphur combine to give black, Lead(II)sulphide, PbS and Lead(IV)sulphide, PbS₂ 

3. Carbon and Oxygen, CO₂ and CO

​Example:

Copper forms two oxides by reacting with oxygen to from copper(I) oxide, Cu₂O and copper(II)oxide, CuO respectively. Analysis of the two oxides shows that in CuO, 11g  of oxygen combines with 89g of copper and in Cu₂O, 20g of oxygen combines with 80g of copper. Show that the results illustrate the law of multiple proportions.

​The Law of Reciprocal Proportion

It states that the masses of several elements A, B, C which combine separately with a fixed mass of another element D, are the same as, or simple multiples of the masses in which A, B and C themselves combine with another.

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