How to work out the no. of moles given the mass of a compound or an element?

Right so, lets say we had 3 grams of hydrogen. We would have 3 moles, why? Because the Atomic Mass of hydrogen is 1 (the little number on the top right of the element/compound). Another example: 8 grams of Oxygen, how many moles? 0.5, why? Because Oxygens mass is 16, and 8 is half of 16 (hence 0.5).