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Heat of Vaporization Example Problem

Worked Example Problems

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Heat of vaporization is the amount of heat energy required to change the state of a substance from liquid to gas. This example problem demonstrates how to calculate the amount of energy required to turn a sample of water to steam.

Problem:

What is the heat in Joules required to convert 25 grams of water into steam? What is the heat in calories?

Useful information: heat of vaporization of water = 2257 J/g = 540 cal/g

Solution:

Part I

Use the formula

q = m·ΔHv

where
q = heat energy
m = mass
ΔHv = heat of vaporization

q = (25 g)x(2257 J/g)
q = 56425 J

Part II

q = m·ΔHf
q = (25 g)x(540 cal/g)
q = 13500 cal

Answer:

The amount of heat required to melt 25 grams of ice is 56425 Joules or 13500 calories.

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