Relative Mass

DEFINITION: 

The average mass of an atom of an element on a scale where an atom of carbon-12 is exactly 12.

HOW TO CALCULATE IT:

''A natural sample of chlorine contains a mixture of 35Cl and 37Cl, whose relative isotopic masses are 35 and 37. 75% of the sample is 35Cl and the rest is 37Cl. Work out the relative atomic mass of chlorine''

(35 X 75) + (37 X 25 )          - ISOTOPIC MASSES X PERCENTAGES

__________________

          100                         -TOTAL PERCENTAGE (75+25)

=35.5

Relative isotopic mass : Mass of an atom of an isoptope of an element on a scale where an atom of carbon-12 is exactly 12.

Relative molecular mass : (Mr) Average mass of a molecule on a scale where an atom of carbon-12 is exactly 12. 

-To work this out add up the RAM of all atoms in the molecule.

Relative formula mass : Average mass of a formula unit on a scale where an atom of carbon-12 is exactly 12

-used for compounds that are ionic or giant covalent.

-To work this out add up the RAM of all the ions in the formula unit.