Relative formula mass

• Relative Formula Mass


  Compounds have a Relative Formula Mass, M
• Find the relative formula mass

  1. Calculate the % mass of an element in a compound

  2. Percentage mass of an element = (mass of that element / mass of the compound) x 100
• Example


  • For NaCO3, M = 23 + 12 + 16 x 3 = 106
• Example


  • A mixture contains 20% iron by mass. What is the mass of iron in 50 g of the mixture? Atomic mass of Fe = 56
• To find the mass of iron


  1. Percentage mass of iron = 20%

  2. Mass of mixture = 50 g

  3. Mass of iron = 50 g x 20% = 10 g
• Iron chloride contains 44.09% iron by mass
• To find the mass of iron chloride needed


  1. Mass of iron needed = 10 g

  2. Percentage mass of iron in iron chloride = 44.09%

  3. Mass of iron chloride = 10 g / 0.4409 = 22.7 g