[govibe]

The question is "The empirical formula of styrene is CH; the molar mass of styrene is 104.14 g/mol. How many H atoms are present in a 2.10 g sample of styrene?"
I converted the 2.1g into moles using the given molar mass... Since there is one mole of H for one mole of CH, it is a one to one molar proportion. I then multiply the number of moles (.020165) by avogadro's number, and I got 1.21 x 10^22 and that is wrong... what did I do wrong?

[Yggdrasil]

You are assuming that each molecule of styrene contains one atom of hydrogen, which is incorrect.  You must do one of two things:

1)  Figure out the molecular formula of styrene using the empirical formula and the given molar mass.

or

2)  Figure out the mass percentage of hydrogen and use that to figure out the number of moles of hydrogen in 2.1g of styrene.

[govibe]

so you always need to figure out the molecular formula if you want to find out the moles of an element in a molecule?

[Yggdrasil]

Yes.  For example, consider glucose (C₆H₁₂O₆).  It has an empirical formula CH₂O.  One mole of glucose will contain 6 moles of carbon, not one mole of carbon.