1. How many joules are needed to change 120.g of
water at 20.0C to steam at 100.0C?
How many joules of energy must be removed
from 126 g of water at 240C to form ice at 0.0C?
How many joules are required to change 225 g
of ice at -15. oC to steam at 140. oC ?

2. How many joules are needed to change 120.g of
water at 20.0C to steam at 100.0C?
First heat H2O from 20.0C to boiling (1000C):
Heat = 120.g x 80.0C x 4.18 J/g0C = J
= 40.1 kJ
Then find heat to vaporize the water:
Heat = 120.g x 2260 J/g = J = 271 kJ
Total heat = 311 kJ

3. How many joules of energy must be removed
from 126 g of water at 240C to form ice at 00C?
First cool water from 240C to 00C:
Heat = 126g x 240C x 4.18 J/g0C = J
(12.6 kJ) = 13 kJ
Then find heat removed to freeze H2O:
Heat = 126 g x 335 J/g = J = kJ
Total heat = kJ
(54.8)

4. How many joules are required to change 225 g
of ice at -15. oC to steam at 140. oC?
First heat the solid (-15. oC) to melting point(00C) :
Heat = 225g x 15.oC x J/g oC = 6851 J
= 6.9 kJ
Then melt the ice:
Heat = 225 g x 335 J/g = J = kJ
Then heat the ice water(0.0C) to boiling(100.0C):
Heat = 225 g x 100.0C x 4.18 J/g0C = J = 94.1 kJ
Then, find heat to vaporize the H2O:
Heat = 225 g x 2259 J/g = J = (508.3) = 508 kJ
Finally, heat the gas (100. oC) to 1400C) :
Heat = 225g x 40.oC x 2.01 J/g oC =18090 J = 18 kJ
Total heat = kJ