# 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