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If you were given a question like below:

What is the mass in grams of a sample of $\ce{Fe2(SO4)3}$ that contains $3.59 \times 10^{23}$ sulfate ions, $\ce{SO4^{2−}}$ ? The molar mass of $\ce{Fe2(SO4)3}$ is $399.91 \, \text{g}/\text{mol}$.

How would you go about solving a question like that?

I'm confused about how to use only the quantity of sulfate ions to find the mass in grams of the whole sample, given it's grams per mole.

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    $\begingroup$ You have to guess, how much full formulas correspond to this number of formula portions. Obviously. $\endgroup$ – permeakra Nov 27 '14 at 18:32
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The formula $\ce{Fe2(SO4)3}$ tells us that 3 molecules of $\ce{SO4^{2-}}$ exist for every $\ce{Fe2(SO4)3}$ molecule.

$\frac{\ce{1 Fe2(SO4)3 molecule}}{\ce{3 SO4^{2-} molecule}} * 3.59*10^{23} \ce{SO4^{2-} molecule}$ can be used to find the number of molecules of ferric sulfate.

You can then use $\frac{1 \ce{mole}}{6.022*10^{23} \ce{molecule}}$ to find the moles of ferric sulfate.

You can then use the given information to solve for grams of ferric sulfate.

Write a comment if there's any ambiguity.

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  • $\begingroup$ Basically the premise behind my approach is abusing conversion factors to get from atoms to grams. If you write everything out first it might be easier to see how the units cancel. $\endgroup$ – Sparkery Nov 27 '14 at 19:01

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