# What mass in grams of iron(III) oxide contains 71.6 g ofiron? Iron(III) oxide is 69.94% iron by mass.

Oct 16, 2016

$\text{102 g}$

#### Explanation:

A substance's percent composition essentially tells you how many grams of each constituent element you get per $\text{100 g}$ of said substance.

In this case, a percent composition of 69.94% means that for every $\text{100 g}$ of iron(III) oxide, ${\text{Fe"_2"O}}_{3}$, you get $\text{69.94 g}$ of iron.

You can thus use this known composition as a conversion factor to take you from mass of iron(III) oxide to mass of iron or vice versa.

Therefore, in order to have $\text{71.6 g}$ of iron, you need to have

71.6 color(red)(cancel(color(black)("g Fe"))) * overbrace(("100 g Fe"_2"O"_3)/(69.94color(red)(cancel(color(black)("g Fe")))))^(color(blue)("known percent composition")) = color(green)(bar(ul(|color(white)(a/a)color(black)("102 g Fe"_2"O"_3)color(white)(a/a)|)))

The answer is rounded to three sig figs, the number of sig figs you have for the mass of iron.