Question

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

Answer 1

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, `"Fe"_2"O"_3`, consists of

• two atoms of iron, `2 xx "Fe"`
• three atoms of oxygen, `3 xx "O"`

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

• two moles of iron, `2 xx "Fe"`
• three moles of oxygen, `3 xx "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 `"55.845 g"` and one mole of oxygen has a mass of `"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 `"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)|)))`