A 69.5 g sample of the compound X3 O4 contains 19.2 g of oxygen atoms. What is the molar mass of element X?

Answer 1

19.2 g O x 1 mol O / 16 g = 1.2 moles O

Since the ratio of moles of Fe to moles of X in the unknown is 4 : 3, we can find moles of X as follows:

1.2 moles O x 3 moles X / 4 mols O = 0.9 moles X

69.5 g - 19.2 g = 50.3 g of X

atomic mass of X = 50.3 g / 0.9 moles = 55.9 g/mol (probably X is Fe, iron)