Poly-atomic molecules have 3 degrees of freedom for rotation and 3 degrees of freedom for translatory motion.
Hence there are 6 degrees of freedom if the atoms are not in a line in the moleculer structure. In CO2 three atoms are in a line, so
CO2 has only 5 degrees of freedom(2 for rotation and 3 for translation). But H2O has 6 degrees of freedom.
Molar heat capacity at constatnt volume is given by
Molar heat capacity at constant pressure is
Hence molar heat capacity of water vapour at constant pressure is 3R+R = 4R