How would you find the mass in grams, if you only know the number of particles of a portion of the formula? 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.
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.