Help! Strange Question about empirical formulas and naming

Question

An oxide of iron is found to be 77.75% iron by mass. Determine the empirical formula for this compound and name it.

Enter the formula with correct capitalization and subscripts (so "H2O" for H₂O). Use all lowercase letters when typing names, no space before parentheses if parentheses are needed (so "tin(II) chloride")

I have tried to solve for the molar mass, but both oxygen and iron equal 1.39. Help?

Answer 1

Unknown oxide of iron: Fe_xO_y

77.75% Fe and 22.25 % O

1) Assume 100 g sample, 77.75 g of Fe and 22.25 g O

2) moles Fe=77.75 g Fe x 1mole Fe/56g Fe = 1.39 moles Fe

moles O=22.25g O x 1mole O/16g O= 1.39 moles O

3) So the mole ratio of Fe:O is 1:1, therefore the empirical formula is FeO (Iron (II) Oxide)

Answer 2

Iron has two ionic states of +2 and + 3. The formula for iron oxide with the Fe + 2 is FeO. The formula for iron oxide with The Fe+3 is Fe2O3. Using the atomic mass of Fe as 55.845 gr/mole and O as 15.999 gr/mole the percent Fe of FeO is 77.73% and of Fe2O3 is 69.94%. To calculate percent mass put the mass of Fe over the total mass of the molecule and multiply by 100. Since 77.73 as calculated is very close to 77.75 I would use FeO as the formula.