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Relative Formula Mass

Compounds have a relative formula mass, M

  • If you have a compound like MgCl2 then it has a relative formula mass, M, which is just the relative atomic masses of all the atoms in the molecular formula added together
  • Calculating relative formula mass is straight forward enough, but things can get a bit more confusing when you start working the percentage compositions of compounds

You can calculate the % mass of an element in a compounds

  • Percentage mass of an element in a compounds = Ar x number of atoms of that element / Mr of the compounds x 100
  • These questions may involve recurring decimals but you will always have a calculator so you should be able to work it out

Calculating relative formula mass

  • The relative formula mass of a substance made up of molecules is the sum of the relative atomic masses of the atoms in the numbers shown in the formula.
  • Relative formula mass has the symbol, Mr. To calculate the Mr for a substance:
  1. work out how many atoms of each element there are in the chemical formula
  2. add together the Ar values for all the atoms of each element present