# An oxide of an element has the formula "X"_2"O"_7 and contains 50.45 % oxygen. What is the atomic mass of the element?

Oct 25, 2014

The atomic mass of the element is 55.00 u.

The formula of the oxide is X₂O₇.

Assume 100 g of the oxide.

Then you have 50.45 g of O and 49.55 g of X.

Moles of O = 50.45 g O × $\text{1 mol O"/"16.00 g O}$ = 3.153 mol O

Moles of X = 3.153 mol O × $\text{2 mol X"/"7 mol O}$ = 0.9009 mol X

Molar mass of X = $\text{49.55 g"/"0.9009 mol}$ = 55.00 g/mol

The atomic mass of X is 55.00 u.