Relative atomic and molecular mass

Core
​•• Define relative atomic mass, Ar, as the average mass of naturally occurring atoms of an element on a scale where the 12C atom has a mass of exactly 12 units.
•• Define relative molecular mass, Mr as the sum of the relative atomic masses. (Relative formula mass or Mr will be used for ionic compounds.)
(Calculations involving reacting masses in simple proportions may be set. Calculations will not involve the mole concept.)

Relative atomic mass is the average mass of an atom of an element, compared to 1/12th the mass of a carbon-12 atom.

Relative molecular mass is the ratio of the average mass of one molecule of an element or compound to 1/12 of the mass of an atom of carbon-12.

The numerical value of Ar is the same as the mass number of the element. Please refer to Periodic Table for Cambridge IGCSE Chemistry 0620 syllabus for the values of the Ar.

For example: Ar of copper, Cu is 64, Ar of Helium, He is 4.

To find Mr, take the Ar of each element and mulitply by the number of atoms of the respective element found in the substance and add all the values together. (This can be used for ionic or covalent compound).

For example:
Mr of Cl2 = 35.5(2) = 71
Mr of NaCl = 23(1) + 35.5(1) = 58.5
Mr of H2O= 2(1) + 16 = 18
CH4= 12+ 4(1) = 16
Mr of Al2O3 = 2(27) + 3(16) = 102
Mr of Ca(OH)2 = 40 + 2(16+1) = 74
Mr of Al(NO3)3 = 27 + 3(14+ 16X3) = 213

Mr of hydrated copper (II) sulfate crystals = CuSO4●5H2O= 64+32+4(16) + 5(2(1) + 16) =250

Find percentage of an element in a compound using Ar or Mr
1)Take the Ar of a particular element multiply by the number of atoms of the element 2) followed by dividing by total Mr of the substance and 3) multiply by 100%

Find the percentage of sodium in NaCl = 23 / 58.5 X 100%= 39.3%
Find the percentage of hydrogen in water = 2(1) /18 X 100% = 11.1%

Find the mass of an element in a compound using Ar or Mr
1)Take the Ar of a particular element multiply by the number of atoms of the element followed by 2) dividing by total Mr of the substance and 3) multiply by total mass

Find the mass of carbon in 20g of methane = 12(1) /16 x 20g = 15.0g
Find the mass of oxygen in 30g of Al2O3 = 3(16) /102 x 30g = 14.1g