Solution
Molar mass of a compound tells you the mass of exactly one mole of said element. Calculate the molar mass of a compound by using
its chemical formula
the molar masses of its constituent elements
In this case a formula unit of iron(III) oxide, Fe
2
O
3
consists of
two atoms of iron, Fe
2
O
3
three atoms of oxygen 3×O
This means that one mole of iron(III) oxide will contain
two moles of iron, 2×Fe
three moles of oxygen, 3×O
Now molar masses of iron and of oxygen are listed as
M
MFe
=55.845gmol
−1
;M
MO
=15.9994gmol
−1
one mole of iron has a mass of 55.845g and one mole of oxygen has a mass of 15.9994g
Use these values to find the mass of one mole of iron(III) oxide
2×55.845g+3×15.9994g=159.6882g
so, if one mole of iron(III) oxide has a mass of 159.6882g, it follows that its molar mass will be
M
MFFe
2
O
3
=159.6882gmol
−1