What mass in grams of iron(III) oxide contains 71.6 g ofiron? Iron(III) oxide is 69.94% iron by mass.
A substance's percent composition essentially tells you how many grams of each constituent element you get per
In this case, a percent composition of
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 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.