Class 11 Laws of Chemical Combinations
LAWS OF CHEMICAL COMBINATIONS
The combination of elements to form compounds is governed by the following five basic laws.
LAW OF CONSERVATION OF MASS
It states that matter can neither be created nor destroyed.
This law was put forth by Antoin Lavoisier in 1789. He performed careful experimental studies for combustion reactions for reaching the above conclusion. This law formed the basis for several later developments in chemistry. In fact, this was the result of exact measurement of masses of reactants and products, and carefully planned experiments performed by Lavoisier.
Lavoisier stated that “during any physical or chemical change the total mass of the products remains equal to the total mass of the reactants”. He showed that when mercuric oxide was heated the total mass of mercury and oxygen produced was equal to the total mass of mercuric oxide.
Can you think of any exception to this law?
Illustration 1: When 20 g of NaH2CO3 is heated, 12.62 g of Na2CO3 and 5.24g of CO2 is produced. How many grams of is produced?
Solution: Total mass of heated = 20 g
Total mass Na2CO3 produced = 12.62 g
Total mass of CO2 produced = 5.24 gms
Mass of H2O produced = 20 – 12.62 –5.24 = 2.14 g
LAW OF DEFINITE PROPORTION
This law was given by, a French chemist, Joseph Proust. He stated that a given compound always contains exactly the same proportion of elements by weight.
Irrespective of the source, a given compound always contains same elements in the same proportion. The validity of this law has been confirmed by various experiments. It is sometimes also referred to as Law of definite composition.
This law implies that irrespective of how a compound is prepared or from where the compound originates, it is always made up of the same elements combined in the same proportion by the weight.
For example, if water is taken from different sources, such as rivers, oceans, wells etc. they all contain hydrogen and oxygen combined in the same proportion by weight.
Illustration 2: When 50 g of ammonia is heated it gives 41.18 g of Nitrogen. When 10 g. of Nitrogen is combined with the required amount of hydrogen it produces 12.14 g show that the given data follows the law of constant compositions.
Solution: If 50 g of NH3 gives 41.18 g of N2, then the percentage of N2 in ammonia is 
If 10g of N2 gives 12.41 gms of Ammonia then percentage of N2 in ammonia is 
LAW OF MULTIPLE PROPORTIONS
This law was proposed by Dalton in 1803. According to this law, if two elements can combine to form more than one compound, the masses of one element that combine with a fixed mass of the other element, are in the ratio of small whole numbers.
For example, carbon and oxygen combine to form CO and CO2. In CO, 12 parts by mass of carbon combine with 16 parts by mass of oxygen while CO2 12 parts by mass of carbon combine with 32 parts by mass of oxygen. Therefore the ratio of the masses of oxygen that combines with a fixed mass of carbon is 16: 32 that is 1: 2.
Illustration 3: Sodium and oxygen combine to form two compounds of which one is Na2O. The percentage of sodium in the other compound is 59%. Find the formula for this compound.
Solution: Percentage of sodium in Na2O is 
and percentage of oxygen is 25.8%. Percentage of sodium in other compound is 59% while that of oxygen is 41%. This means that in the first compound (Na2O) if we take 100 gm then 25.5 gm of oxygen will be present therefore the mass of sodium combining with 1g of oxygen would be 
Similarly, in the second compound, the mass of sodium combining with one g of oxygen is 59/41= 1.44 g.The ratio of masses of sodium combining with the fixed mass of oxygen is 2.87: 1.44 = 2: 1. Therefore formula of the other compound is Na2O2.