how many molecules of element in sample
Avogadro's number is 6.02*10^23 atoms or molecules per mole. One mole of oxygen is 16 grams, so one mole of oxygen gas is 32 grams. If one mole of oxygen gas is 32 grams, then 16 grams is equal to half a mole of oxygen gas. The number of molecules in half a mole is equal to half of Avogadro's number.