Law of conservation of mass
This law was stated by Russian scientist Mikhail Lomonosov in 1756.
Consider a reaction of hydrogen and oxygen, we get water as the product.
According to the law, the sum of masses of reactants (hydrogen and oxygen) is always equal to the sum of masses of product (water).
Law of a definite combination
This law was stated by French chemist Joseph Proust. He was the first scientist to think that chemical compounds are formed from definite proportions.
Joseph Proust did study copper carbonate to prove this law. He compared naturally occurred copper carbonate and artificial copper carbonate.
He found that proportions of copper, carbon, and oxygen of artificial copper carbonate were exactly identical to that of naturally occurring copper carbonate.
This law works well for stoichiometric compounds but fails with non-stoichiometric compounds.
Law of multiple proportions
This law was stated by the British scientist who introduced atomic theory, John Dalton. This law was published in the first part of the first volume of the New System of Chemical Philosophy.
As we know carbon reacts with oxygen to form two compounds – carbon monoxide (CO) and carbon dioxide (CO2).
A fixed mass of carbon says 100 grams may react with 266 grams of oxygen to form carbon dioxide. On the other hand, carbon reacts with 133 grams of oxygen to form carbon monoxide.
The ratio of masses of oxygen that can react with carbon is 266:133 or you can say 2:1 which are small whole numbers.
This law works well for stoichiometric compounds but fails with non-stoichiometric compounds (compounds in which the proportion of elements cannot be represented by integers). Also, it does not work for polymers.