The enthalpy of 1 lb of water at 212 F is 180.07 Btu/lb, While the enthalpy of 1 of steam at 212 F is 1150.4 Btu/lb, so that the enthalpy (latent heat) of vaporization is (1150.4 – 180.07) 970.3 Btu/lb, the latter being the total quantity of enthalpy required to vaporize 1 lb of water at 212 F into steam at 212 F. Of the total energy supplied, the amount required to do the work of the constant pressure expansion is approximately