Question: how many moles of oxygen are contained in 64.00 grams of oxygen?
Answer: molar mass of oxygen is 16 grams, it means that 1 mole oxygen weighs 16 grams. therefore 64 gram oxygen = 64/16 = 4 mole oxygen. 1 mole of any element contains 6.022 * 10^23 atoms in it. hence 4 moles of oxygen.