Use these guideline for converting to grams, moles or Avocado's:
1. Generally you convert moles to gram for calculating the amount of a reacting necessary to come up with a certain amount of product in a reaction.
2. You do moles conversion when you have to prepare a solution of certain Molarity. For instance how many moles of HCl is required to prepare 0.5 molar concentration solution.
3. Avogagdro's is a number of molecules or atoms present Ina mole of a substance. So you convert to Avocado's number when you are asked to find out the number of atoms or molecules present in a quantity of a substance or a compound or an element.