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

1 Answer
Stefan V.
Oct 16, 2016

#"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.

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