Question

What is the mass of iron contained in 100g of an iron ore?

Answer 1

It depends on which iron ore it is.
The most common form is Fe2O3
Molar mass o iron =  55.85
Molar Mass of Oxygen = 16 

So mass of iron i Fe2O3  = (2*55.85)  * 100/ (2*55.85 + 3*16) = 69.9 g