How many moles in 32g of oxygen gas?

The number of grams in a mole of a molecule is found by looking at the Atomic Mass of each element on the Periodic Table. The molar mass of an element is it's atomic mass in grams. The atomic mass of oxygen is 15.999. Since there are two atoms of oxygen in a molecule of oxygen, the molar mass of oxgen (O2) is 15.999 x 2. That comes out to 31.998g/mol, or you can round to 32 if significant figures arn't important to you.