How relative atomic masses were determined using the law of chemical combinations and the compounds formed?please give examples

Atomic masses listed on the periodic table are more accurately referred to as the relative atomic mass.  This is because the atomic mass listed on the periodic table is an average of the atomic masses of all of the naturally occurring isotopes of the particular element.  For example, carbon has isotopes C-12, C-13, and C-14.  This means that the mass of each of these atoms is different because they all have a different number of neutrons (same number of protons) and neutrons have a mass that contributes to the overall mass of the atom. Scientists have calculated the average atomic masses using the percentage of each isotope typically found.  The carbon 12 isotope is most common and that is why the mass is close to 12. 

To find the atomic mass of a compound, you have to take into account the atomic mass of each individual element.  You simply add the mass of each of the components to each other.  Each unique compound has a different number of atoms of each element, so the atomic mass will vary.  For example:

H2O: 1.01 + 1.01 + 16.00 = 18.02

H2O2:  1.01 + 1.01 + 16.00 + 16.00 = 34.02

Although these two compounds have the same elements, they have a different combination so they are different compounds which means they will have a different mass.