What is the first law of …

Law of Chemical Combination

(1) Law of Conservation of Mass

In a chemical change, total mass remains conserved. i.e. mass before the reaction is equal to mass after the reaction

H2​(g)+Cl2​(g)⟶2HCl

Mass before reaction=2+71=73g

Mass after reaction=2x36.5=73g

(2) Law of constant or Definite Proportions 

All Chemical compounds are found to have constant composition irrespective of their method of preparation or source.

For g. in water (H2​O), hydrogen and oxygen combine in 2:1 molar ratio, this ratio remains constant whether it is tap water or river water.

(3) Law of Multiple Proportions

When one element combines with the other elements to form two or more different compounds, the mass of one element which combines with a constant mass of the other, bear a simple ratio to one another. 

For eg. C and O2​ reacts together to form CO and CO2​. Here, fixed mass of carbon (12g) reacting with different masses of oxygen (16g and 32g) . Therefore, ratio of the masses of oxygen has ratio 16:32 or 1:2