Question

How much heat in joules is needed to vaporize an 18.0 gram sample of water at 100c?

Answer 1

The water is already at its boiling point
`(T=100^(\circ)C)`, 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

`Q=m L_v`
where
m is the mass of the water
Lv is the latent heat of evaporation

The latent heat of evaporation of water is
`L_v = 2265 J/g`, and the mass of this sample of water is
`m=18.0 g`, therefore the heat needed to vaporize the water is

`Q=mL_v = (18.0 g)(2265 J/g)=4.08 \cdot 10^4 J =40.8 kJ`