Relative formula mass

Relative atomic mass (Ar)

Way of saying how heavy atoms are in comparison with carbon 12

Ends up being the same as the mass number

For example helium: mass number = 4 so relative atomic mass = 4.

Relative formula mass (Mr)

Relative formula masses are just the relative atomic masses of a compound (e.g. magnesuim chloride) added together

MgCl2

Mg = 24

Cl2 = (35.5 * 2)= 71

A

A relative formula mass (Ar or Mr) of a substance in grams = one mole of that substance.

Moles

To convert between moles and grams...

Mass in grams (of element or compound)

Mr (of element or compound)

= Number of moles

New Page