# How would you find the mass in grams, if you only know the number of particles of a portion of the formula? [closed]

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.

• You have to guess, how much full formulas correspond to this number of formula portions. Obviously. – permeakra Nov 27 '14 at 18:32

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.