How much heat is necessary to vaporize 500 g of water at its boiling point?
It depends on what temperature the water is starting at.
If the water is in solid form it would go through 2 phase changes. That equation is:
However, when calculating the amount of heat necessary to heat to a certain temperature you need the initial temperature. That equation looks like this:
We're lacking some information in the question. Get that temperature information and we can figure it out.