At STP, which sample contains the same number of molecules as 11.2 liters of CO2 (g) at STP?

(1) 5.6 L of NO2 (g) (3) 11.2 L of N2 (g)

(2) 7.5 L of H2 (g) (4) 22.4 L of CO (g)

The answer is (3) 11.2 L of N2 (g). When the choices are all gases, the number of molecules depends on the molar number of the gas. Under the condition of STP, the molar number of a gas is depends on volume only. So the same volume of gas has the same number of molecules.