What is the percent of oxygen by mass in a pure sample of fecr2o4?

Answer is: percent of oxygen by mass in FeCr ₂O₄ is 28,5%.

Mr (FeCr₂O₄) = Ar (Fe) + 2·Ar (Cr) + 4·Ar (O)

Mr (FeCr₂O₄) = 55,85 + 2·52 + 4·16

Mr (FeCr₂O₄) = 223,85.

ω (O) = 4·Ar (O) : Mr (FeCr₂O₄)

ω (O) = 64 : 223,85

ω (O) = 0,285 = 28,5%.

FeCr₂O₄ - iron chromium oxide or iron chromite.