Who gave the two Laws of Chemical Combination? Write down the statements of these
laws.

Solution
1.Law of conservation of mass was given bty Lavoisier i  1774
2.Law of constant proportion was given by Joseph proust in 1779

Law of conservation of mass :-
Mass can neither be created nor destroyed in a chemical reaction. It means that the sum of the masses of the reactants and the products remains the same during a reaction.
Example:-
If 100gm of calcium carbonate is heated then, by the law of conservation of mass,
sum of reactants =   sum of the products;
Calcium carbonate(100g)    −→heat→heat calcium oxide(56g) + carbon dioxide(44g)
sum of reactants = 100g
sum of products= 56g+ 44g
                         =100g

 

The law of constant proportion is also known as the law of definite proportion. This law, which was introduced by Proust, stated that ‘in a compound, the elements are always present in definite proportions by mass’.

For example, pure water obtained from any source (well, river, lake,) and from any part of the world will always contain two hydrogen atoms and one oxygen atom. 

Hydrogen and oxygen respectively combine together in the ratio of 1:8 by mass to form water. The ratio by the number of atoms for water will always be H : O = 2 : 1. Thus, 18 g of water contains 2 g of hydrogen and 16 g of oxygen, or the ratio of masses of hydrogen to oxygen will always be 1:8.