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 has two main isotopes with mass numbers 35 and 37
• The relative atomic mass of chlorine is 35.5 due to the weighted average of the two isotopes
• Calculating relative formula mass

  1. Multiply the relative atomic mass of each element by the number of atoms of that element in the formula

  2. Add up all the products
• Relative formula mass calculations

  • Methane (CH4): 1 x 12 + 4 x 1 = 16
  • Calcium sulfate (CaSO4): 1 x 40 + 1 x 32 + 4 x 16 = 136
  • Magnesium hydroxide (Mg(OH)2): 1 x 24 + 2 x 16 + 2 x 1 = 58