You heat 3.869 g of a mixture of fe3o4 and feo to form 4.141 g fe2o3. What is the mass of oxygen reacted?

The reaction is

FeO + Fe3O4 + 1/2 O2---> 2Fe2O3

Thus as shown in the balanced equation two moles of Fe2O3 are formed when 0.5 moles of O2 reacted with mixture of FeO and Fe3O4

moles of Fe2O3 = MAss / Molar mass = 4.141 / 159.69 = 0.0259 moles

So moles of O2 needed = 0.5 X 0.0259 = 0.01295

Mass of O2 = moles X molar mass = 0.01295 X 32 = 0.4144 grams