We heat 1.00 g of Iron and the product has a mass of 1.28 g, so the mass of the iron oxide is 1.28 g. It has two components, one is Iron and the other one is Oxygen. So, if the mass of the Iron was 1.00 g and the mass of the oxide is 1.28 g, 0.28 g are from the oxygen. The empirical formula maybe represented like this:
FexOy
mass of FexOy = 1.28 g
mass of Fe = 1.00 g
mass of O = 0.28 g
To find the empirical formula we have to find the number of moles of each element. To do that we will use the molar mass of them:
molar mass of Fe = 55.85 g/mol
molar mass of O = 16.00 g/mol
x = number of moles of Fe = 1.00 g / (55.85 g/mol) = 0.0179 moles
x = number of moles of Fe = 0.0179 moles
y = number of moles of O = 0.28 g /(16.00 g/mol) = 0.0175 moles
y = number of moles of O = 0.0175 moles
So according to the moles, the relationship between these compounds are:
Fe₀.₀₁₇₉O₀.₀₁₇₅
The last step to find the empirical formula of our compound is to divide those numbers by the smallest one.
x = 0.0179/0.0175 = 1 x = 1
y = 0.0175/0.0175 = 1 y = 1
So the formula of our compound is :
FeO
Since the oxidation number of O in the oxides is always -2, the oxidation number of Fe is +2.
The oxidation number of Fe is +2.