Question

a compound contains 69.94 percent iron and 30.06 percent oxygen. what is its molecular formula if the molar mass of the compound is 199.55 grams per mole?

Answer 1

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