The equation to be used to calculate the heat required to vaporize 1 gram of water at 100 degrees eclsius and 1 atmosphere is the enthalpy. It is represented as H = mCpT where H is the heat released after phase change, m is the mass of the substance, Cp is the specific heat capacity of the substance and T is the change in temperature.
H = mCpT
H = (1g/1000g/kg)(4.184kJ/kg-K)(100 - 20)
H = 0.335 kJ