Another one of my answer that needs correcting is this:
When H2 is mixed with Iron III oxide at 400*C, black remnants appear: a compound that is 72.3% Fe and 27.7% O by mass. What is the empirical formula for the black residue?
Is the empirical formula is FeO?
Fe = 72.3g / 56.0g mole = 1.29 mole
O = 27.7 / 16.0 g mole = 1.73 mole
Fe = 1.29 / 1.73 = .7
O = 1.73/ 1.73 = 1
.7 is wrong, right?