Relative Formula Mass

• Such as is used to introduce examples
• To find the relative formula mass of a compound, add together the relative atomic masses of all the atoms in the molecular formula
• Find the relative formula mass of MgO


  • Look up the relative atomic masses of all the elements in the compound on the periodic table
  • Add up all the relative atomic masses of the atoms in the compound
• Relative atomic mass


  The mass of an atom relative to the mass of a carbon-12 atom
• Find the relative formula mass of Ca(OH)2


  • Look up the relative atomic masses of all the elements in the compound on the periodic table
  • Add up all the relative atomic masses of the atoms in the compound
• The small number 2 after the bracket in the formula Ca(OH)2 means that there are two of everything inside the brackets
• Calculating percentage mass of an element in a compound

  1. Look up the relative atomic masses of all the elements in the compound on the periodic table

  2. Add up the relative atomic masses of all the atoms in the compound to find the relative formula mass

  3. Use the formula: Percentage mass = (Relative atomic mass x Number of atoms of that element) / Relative formula mass x 100
• Find the percentage mass of sodium (Na) in sodium bromide (NaBr)

  • Look up the relative atomic masses of all the elements in the compound on the periodic table
  • Add up the relative atomic masses of all the atoms in the compound to find the relative formula mass
  • Use the formula: Percentage mass = (Relative atomic mass x Number of atoms of that element) / Relative formula mass x 100