The mass percent of iron in an iron oxide is 77.7%. Find the empirical formula.

Question

Quizplus · Accepted Answer

Since the mass percent of iron is 77.7%, we can assume that 77.7 grams out of 100 grams of the compound are due to iron. We can calculate the mass percent of oxygen by subtracting the mass percent of iron from 100. Therefore, the mass percent of oxygen is 22.3%. 
To find the empirical formula, we need to convert the mass percent to moles. Assuming a 100-gram sample of the compound, we have 77.7 grams of iron and 22.3 grams of oxygen. 
Moles of iron = mass of iron/molar mass of iron = 77.7 g/55.845 g/mol = 1.391 moles
Moles of oxygen = mass of oxygen/molar mass of oxygen = 22.3 g/15.999 g/mol = 1.394 moles
Since the mole ratio between iron and oxygen is approximately 1:1, the empirical formula is FeO.

The Mass Percent of Iron in an Iron Oxide Is