Question

moles-?

Answer 1

so you take the given, in this case 38.1 grams of carbon, and you do dimensional analysis.

you need to set the the problem up so that you can cancel like terms grams to grams, moles to moles - in this case the 38.1 grams of C can be divided by the molar mass (from the periodic table) of carbon. What ever the molar mass of that element is equivalent to one mole of that element. You divide on bottom and multiply on top.

38.1 g of C divided by 12.01 g C (molar mass) multiplied by the number of moles (in this case one) --- your ending term is in moles (the grams cancelled each other out). You can get 3.17 moles of C from 38.1 (also answer is three significant figures because your original problem has three sig fig's).
Hope that helped. -- See YouTube videos on the subject there are some really good ones out there!