3. How many calories will it take to boil 100 g of water if the water is already at 100 ^oC (the boiling point of water)?

In this problem, all we have to worry about is the mass. Remember that no change of temperature occurs during the boiling of the water. Multiply the mass times the heat of vaporization of water so that the units of grams cancel out. Wow...that is a lot of calories!