How much energy does it take to vaporize water at the temperature of boiling?

Question

Given that the heat of vaporization of water is 2256.4 kJ/kg, how much energy (kJ) does it take to vaporize 200.0 g of water that is already at the temperature of boiling.

Answer 1

Hello, Margaret,

The units of the specific heat constant for vaporization tell you exactly what to do. 2256.4 kJ/kg is saying "it takes 2256.4 kiloJoules to vaporize 1 kg of water." We have 200.0 grams of water, which can be converted to kg by dividing by 1000.

Look at the units, and we can see that if we multiply the two, that kg will cancel, leaving just kiloJoules.

(2256.4 kJ/kg)*(0.200 kg) = 451 kiloJoules

Bob