7. How many joules of energy do I need to change 2 grams of water from 0°C solid water to 0° liquid water? A. 0 joules
B. 334 joules
C. 167 joules
D. 668 joules

8. How many grams of water can I covert from a liquid to a gas with 6768 joules?
A. 334 grams
B. 3 grams
C. 1 gram
D. 2,256 grams

7. D. 668 joules

8. B. 3 grams

Given

2 grams of water

6768 joules of energy

Required

Energy

Mass of water

Solution

The heat to change the phase can be formulated :

Q = m.Lf (melting/freezing)

Q = m.Lv (vaporization/condensation)

Lf=latent heat of fusion

Lv=latent heat of vaporization

7. phase change (ice to water)⇒Heat of fusion water=334 J/g

Q = 2 g x 334 J/g

Q = 668 J

8. phase change(water to vapor)⇒heat of vaporization water=2260 J/g

6768 J = m x 2260 J/g

m = 3 g

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