Relative Formula Mass

• Compounds have a relative formula mass, Mr
• Example

  Find the relative formula mass of MgCl2

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

  > Ar of Mg = 24 and the Ar of Cl = 35.5

  2) Add up all the relative atomic masses of the atoms in the compound

  > Mg + (2 x Cl) = 24 + (2 x 35.5) = 95. So Mr of MgCl2 = 95
• You can find the relative atomic mass (Ar) of an element from the periodic table - it's the same as its mass number
• (Ar x number of atoms of that element / Mr of the compound) x 100 =

  Percentage mass of an element in a compound
• Example

  Find the percentage mass of sodium in sodium carbonate, Na2CO3

  > Ar of sodium = 23, Ar of carbon = 12, Ar of oxygen = 16

  > Mr of Na2CO3 = (2 x 23) + 12 + (3 x 16) = 106

  > Percentage mass of sodium = (23 x 2) / 106 x 100 = 43%
• Example

  A mixture contains 20% iron ions by mass. What mass of iron chloride (FeCl2) would you need to provide the iron ions in 50 g of the mixture?

  1)Find the mass of iron in the mixture

  > The mixture contains 20% iron by mass, so in 50 g there will be 50 x (20 / 100) = 10 g of iron

  2) Calculate the % mass of iron in iron chloride

  > % mass of iron = 56 / [56 + (2 x 35.5)] x 100 = 44.09...%

  3) Calculate the mass of iron chloride that contains 10 g of iron

  > Iron chloride contains 44.09% iron by mass, so there will be 10 g of iron in 10 / (44.09... / 100) = 23 g

  = So you need 23 g