Question

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

Answer 1

`"102 g"`

Explanation:

A substance's percent composition essentially tells you how many grams of each constituent element you get per `"100 g"` of said substance.

In this case, a percent composition of `69.94%` means that for every `"100 g"` of iron(III) oxide, `"Fe"_2"O"_3`, you get `"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 `"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.