Relative Formula Mass

• Relative formula mass (Mr) is the relative atomic masses of all the atoms in a molecular formula added together
• To find the relative formula mass:

  1. Look up the relative atomic masses of all the elements in the compound on the periodic table.
  2. Add up all the relative atomic masses of the atoms in the compound.
• Percentage mass of an element in a compound = (relative atomic mass (Ar) x numbers of that element / Mr of the compound) x 100
• Relative atomic mass (Ar) is user instead of a mass number when referring to an element as a whole.

  This is an average mass taking into account the different masses and abundances (amounts) of all the isotopes that make up an element.