Law of conservation of mass - No detectable gain or loss in mass occurs in chemical reactions. However, the state of a substance may change in a chemical reaction. For example, substances involved in a chemical reaction can change from solid states to gaseous states but the total mass will not change. Note that the energy released (exothermic) or adsorbed (endothermic) in a chemical reaction is a result of energy transfer between atoms and their environment.




Law of Definite Proportion - The elements in a given compound are always combined in the same proportion by mass. This law forms the basis for the definition of a chemical compound. For example, a water molecule (H2O) consists of two hydrogen atoms each of relative mass of 1 and one oxygen atom of relative mass of 16 (rounded to nearest integer number). 


By putting a sensible unit measurement this means that there are 2 g of hydrogen and 16 g of oxygen in a sample of 18 g of water. The ratio is 1 to 8. So, the sum of ratio is 9.


 Eg


I. 100 grams O2 + 1 grams H2 → 9 grams H2O + 92 grams O2 
II.  50 grams O2 + 5 grams H2 → 45 grams H2O + 10 grams O2
III.  80 grams O2 + 5 grams H2 → 45 grams H2O + 40 grams O2


How can we determine the amount of water and oxygen are produced ?
Explanation :


Remember the ratio of water, 1 Hydrogen and 8 Oxygen.
That means, 
1 grams H2 that reacted multiplied to 9 ( the sum of ratio)
And than the sum of mass that reacted subtracted with the multiplication operation


Sum of mass 100 grams O2 added 1 grams H2 equal to 101 grams. The Hydrogen multiplied by 9 (sum of ratio), and we get 9 for Hydrogen. Sum of mass (101) subtracted by 9, it become 92.