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Calculating the percentage mass of an element in a compound

Calculating the percentage mass of an element in a compound

  • The percentage is the: Ar x number of atoms for that element in the formula / the Mr x 100
    • E.g. MgCl2 (Mg Ar = 24         Cl Ar = 35.5)
      • Mr = 24 +(35.5 x 2) = 95
      • The percentage that is chlorine is:
        • 5 x 2 / 95 x 100 = 74.7%

You can use the percentage masses to work out the empirical formula of a compound

  • Step 1: List the elements
  • Step 2: Write their percentage or experimental masses
  • Step 3: Divide each number by the Ar of that element
  • Step 4: Turn the numbers you get into a ratio
  • Step 5: in the simplest form that is the number of each element
    • g Find the formula for iron oxide where 44.8g of iron react with 19.2g of oxygen (Ar = iron = 56 and O = 16)
    • Step 1:  Fe                                          O
    • Step 2: 8                                        19.2
    • Step 3 8/56 = 0.8                      19.2 / 16 = 1.2
    • Step 4: x10 = 8                                  x10 = 12
    • Step 5: 8:12        2:3          so…
      • This method is based on experiment to work out the formula for a compound
      • It is the only way of doing it
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