**The number of molecules in 16 grams of oxygen gas is 3.01*10^23, equal to half of Avogadro's number.** One mole of oxygen gas is approximately 32 grams. Sixteen grams of oxygen gas is half a mole.

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. Sixteen grams of oxygen gas contains 3.01*10^23 molecules.