Question

Question #1b172

Answer 1

You need `color(blue)("5.86 mol of O"""_2)`.

Explanation:

Your balanced equation is

`"4Fe" + "3O"_2 → "2Fe"_2"O"_3`

This tells you that `"2 mol of Fe"""_2"O"_3` require `"3 mol of O"""_2`.

This is called the molar ratio of `"Fe"_2"O"_3` to `"O"_2`.

You want to convert the given moles of `"Fe"_2"O"_3` to moles of `"O"_2`.

We can use the molar ratio to create a conversion factor.

It will be either `("2 mol Fe"""_2"O"_3)/("3 mol O"""_2)` or `("3 mol O"""_2)/("2 mol Fe"""_2"O"_3)`.

We choose the one that will give us the correct units in the answer.

It will be the second one. Then

`3.91 color(red)(cancel(color(black)("mol Fe"_2"O"_3))) × ("3 mol O"""_2)/(2 color(red)(cancel(color(black)("mol Fe"_2"O"_3)))) = "5.86 mol O"""_2`

Notice that the units cancelled to give the desired units of `"mol O"""_2"`.

If we had chosen the other conversion factor, we would not have gotten the correct units.