Determine the mass, in grams, of a quantity of the amino acid lysine, containing 3.13 mol N atoms.
i tried a few different things but none of them seem to give me the right answer.
Since you have 3.13 moles of nitrogen atoms and there are 2 nitrogen atoms per molecule, there must be $\displaystyle \frac{3.13}{2} = 1.565$ moles of lysine.
The molar mass of lysine is 146.188 g/mol (from wikipedia, use your own period table for this value).
Simply multiply them together and you should get the amount of lysine in grams.