Relative formula mass

• Relative formula mass


  The sum of the relative atomic masses of the atoms in the numbers shown in the formula
• Relative formula mass has no units
• Relative formula mass never involves big numbers
• Relative atomic mass


  The average mass of the isotopes of an element weighted to take into account the abundance of each isotope
• Chlorine isotopes


  • Chlorine-35
  • Chlorine-37
• Chlorine-35 is more abundant than Chlorine-37
• The periodic table shows the average relative atomic mass of chlorine as 35.5
• Calculating relative formula mass


  Sum the relative atomic masses of the atoms in the numbers shown in the formula
• Methane


  • 1 x Carbon (12)
  • 4 x Hydrogen (1)
  • Relative formula mass = 16
• The relative formula mass is the same whether there is 1 molecule or 2 molecules of the compound
• Magnesium hydroxide


  • 1 x Magnesium (24)
  • 2 x Oxygen (16)
  • 2 x Hydrogen (1)
  • Relative formula mass = 58