The water is already at its boiling point
, therefore we don't need additional heat to increase its temperature. Instead, we have to give the heat necessary to make it evaporate. The energy needed is equal to
where
m is the mass of the water
Lv is the latent heat of evaporation
The latent heat of evaporation of water is
, and the mass of this sample of water is
, therefore the heat needed to vaporize the water is