# How do you find the molar mass of "Fe"_2"O"_3?

Jul 15, 2016

Here's how you can do that.

#### Explanation:

As you know, the molar mass of a compound tells you the mass of exactly one mole of said element. You can 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, ${\text{Fe"_2"O}}_{3}$, consists of

• two atoms of iron, $2 \times \text{Fe}$
• three atoms of oxygen, $3 \times \text{O}$

This means that one mole of iron(III) oxide will contain

• two moles of iron, $2 \times \text{Fe}$
• three moles of oxygen, $3 \times \text{O}$

Now, the molar masses of iron and of oxygen are listed as

M_("M Fe") = "55.845 g mol"^(-1)

M_("M O") = "15.9994 g mol"^(-1)

This tells you that one mole of iron has a mass of $\text{55.845 g}$ and one mole of oxygen has a mass of $\text{15.9994 g}$. Use these values to find the mass of one mole of iron(III) oxide

overbrace(2 xx "55.845 g")^(color(blue)("the contribution of 2 moles of Fe")) + overbrace(3 xx "15.9994 g")^(color(purple)("the contribution of 3 moles of O")) = "159.6882 g"

So, if one mole of iron(III) oxide has a mass of $\text{159.6882 g}$, it follows that its molar mass will be

M_("M Fe"_2"O"_3) = color(green)(|bar(ul(color(white)(a/a)color(black)("159.6882 g mol"^(-1))color(white)(a/a)|)))