The percentages are given in weight. You need to transform them into moles.
Take a basis of 100 grams of compound. Then there are 69.94 grams of iron and 30.06 grams of oxygen.
Use the atomic masses of iron and oxygen to obtain each number of moles.
Molar mass of iron: 55.85 g/mol
# moles of iron = 69.94 g / 55.85 g/mol = 1.252
Molar mass of oxygen: 16 g/mol
# moles of oxygen = 30.06 g / 16 g/mol =1.87875
Proportions
1.87875 moles of Oxigen/ 1.252moles of iron = 1.50 /1
3moles of Oxygen: 1 mol of Iron
Empirical formula: FeO3
Molar mass of the empirical formula: 55.85 g/mol + 3(16g/mol) = 103.85 g/mol
Molar mass of the compound / molar mass of the empirical formula = 199.55 g/mol / 103.85 g/mol = 1.92
Round to 2.
Then the molecular formula is the empirical formula times 2.
Fe2O6
