Avogadro's number of 6.02E23 is defined as the number of molecules in a mole. (For a compound, it would be the number of formula units in a mole.) Therefore, if we are given 15 moles of oxygen gas, multiplying by 6.02E23 gives the total number of molecules in the gas cylinder. This is equivalent to 9.03E24 molecules of oxygen gas contained in the cylinder.
