The combination of elements to form compounds is governed by the following five basic laws.
It states that matter can neither be created nor destroyed.
This law was put forth by Antoine Lavoisier in 1789. He performed careful experimental studies for combustion reactions for reaching to the above conclusion. This law formed the basis for several later developments in chemistry. Lavoisier stated that “during any physical or chemical change the total mass of the products produced is equal to the total mass of the reactants reacted”. 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.
Ex. 1: When 20 g of NaHCO3 is heated, 12.62 g of Na2CO3 and 5.24g of CO2 is produced. How many grams of H2O is produced?
Solution: Total mass of NaHCO3 heated = 20 gms
Total mass Na2CO3 produced = 12.62 gms
Total mass of CO2 produced = 5.24 gms
Mass of H2O produced = 20–12.62 –5.24 = 2.14 gms
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 difference sources, such as rivers, oceans, wells etc. they all contain hydrogen and oxygen, combined in the same proportion by weight in it.
Ex. 2: When 50 g of ammonia is heated it gives 41.18 g of Nitrogen. When 10 g of Nitrogen is combined with required amount of hydrogen it produces 12.14g ammonia. Show that the given data follows the law of constant compositions.
Solution: If 50g of Ammonia gives 41.18g of Nitrogen, then the percentage of Nitrogen in ammonia
41.18/50 *100= 82.3%
If 10g of Nitrogen gives 12.14 g of Ammonia then percentage of Nitrogen in ammonia is
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 combines with 16 parts by mass of oxygen while in CO2 12 parts by mass of carbon combines 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.
Ex. 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 of this compound.
Solution: Percentage of sodium in Na2O is 2/23*100= 74.2% 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 25.8 gm of oxygen will be present therefore the mass of sodium combining with 1g of oxygen would be 74.2/25.8= 2.87 g. Similarly in the second compound the mass of sodium combining with one gm of oxygen is 59/41 = 1.44g. 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.
This law which was proposed by Ritcher (1792) states that “when two elements combine separately with fixed mass of third element then the ratio of their masses in which they do so is either the same or some whole number multiple of the ratio in which they combine with each other”.
Carbon, Sulphur and Oxygen form CO2, SO2 and CS2. In CO2 12 parts by mass of carbon combine with 32 parts by mass of oxygen while in SO2 32 parts by mass of Sulphur combine with 32 parts by mass of oxygen. Ratio of masses of carbon and sulphur which combine with fixed mass of oxygen is 12:32 or 3:8.In CS2 12 parts by mass of carbon combines with 64 parts by mass of sulphur therefore the ratio of mass of carbon to sulphur in carbon disulphide is 12:64 i.e. 3:16.
Therefore, the ratio is 3/8:3/16 or 2:1
This law which was proposed by Gay – Lussac states that, the volumes of gaseous reactants reacted and the volumes of gaseous products formed, all measured at the same temperature and pressure bear a simple ratio.
The reaction involved in Haber’s Process (Nitrogen and hydrogen gases react to form ammonia.It is observed that the ratio of the volumes of N2 and H2 reacted and volume of NH3 produced is equal to 1:3:2 which is a simple ratio.