Question: at stp how many molecules of water vapour are in 22.4l?
Answer: At STP 1 mole of an ideal gas = 22.4 liters. Assuming that water vapor behaves as an ideal gas, then... 0.08929 moles H2O x 6.02x1023 molecules/mol = 5.4x1022 molecules = 5x1022 molecules (1 sig. fig.)