Question

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?

Answer 1

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 × `"1 mol O"/"16.00 g O"` = 3.153 mol O

Moles of X = 3.153 mol O × `"2 mol X"/"7 mol O"` = 0.9009 mol X

Molar mass of X = `"49.55 g"/"0.9009 mol"` = 55.00 g/mol

The atomic mass of X is 55.00 u.