Avogadro's law applies at STP where P is 1 atm and T is 273K. From Avogadro's law; 1 mole of gas takes up 22.4 L of volume at STP. I. e 1 moles = 22.4 L. Hence 1.2 moles of water vapor will take up. 1.2 * 22.4 = 26.88L. Or using ideal ga s eqn PV = NRT. We have P = 1 atm. N = 1.2 moles. R = 0.0821 L and T =273 K. So V = NRT/P.Then we have 1.2 * 0.0821 * 273 = 26.88L.
