Do not worry that you find these problems difficult; many students consider them to be the toughest problems in the course.

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In general, you just have to keep thinking about where the energy comes from and where it goes. For example, if a hot object is dropped in a calorimeter, it will cool down. Thus, it loses energy. This means that its q, calculated from q=(m)x(c)x(deltaT), is negative.

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Since the object loses energy, the calorimeter and the liquid must BOTH gain energy. Thus, their qâ€™s, q=(m)x(c)x(deltaT), will both be positive. The sum of their qâ€™s, then, must be equal to the negative of the metalâ€™s q, since energy cannot be created or destroyed.Â Thus, in the calorimeter problems, you know

Â -q_{object}=q_{liquid}Â + q_{calorimeter}

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In any given problem, you will be able to calculate 2 of the 3 qâ€™s with the information given. Thus, you can figure the third q and then use that to calculate whatever you need to find.

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Here are a few more problems. The answers are at the very bottom.

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1. 75 grams of ethanol (c=2.4 J/gC) is initially at 25.0 C. If 1625 J of energy are REMOVED from the ethanol, what is the new temperature?

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2. 121 cal of heat are added to 50.0 g of water initially at 15.0 C. What is the final temperature of the water?

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3. How much heat is necessary to raise the temperature of a 15.0 g metal (c=0.456 cal/gC) from 15.0 C to 25.0 C?

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4. A 500.0 g mass of unknown metal at 100.0 C is dropped in a calorimeter initially at 25.0 C (c for the calorimeter=3.77 cal/gC, m for calorimeter is 5.0 g). There are 120.0 g of water in the calorimeter. The final temperature of the water, calorimeter, and metal is 37.0 C. What is the c of the metal?

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5. A 50.0 g chunk of metal (c=0.121 cal/gC) is heated to 60.0 C. It is then dropped into a calorimeter (c for the calorimeter=3.77 cal/gC, m for calorimeter is 5.0 g) that contains 100.0 g of an unknown liquid. The temperature of the liquid raises from 15.0 C to 20.0 C. What is the c of the liquid?

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6. A 150.0 g piece of metal (c=0.0560 cal/gC) is dropped into a calorimeter which contains 40.0 g of water. The metal was initially at 55.0 C and the calorimeter was initially at 25.0 C. The mass of the calorimeter is 5.0 g. If the temperature of the calorimeter increased to 29.0 C, what is the c of the calorimeter?

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ANSWERS:

1. 16 C

2. 17.4 C

3. 68.4 cal

4. 0.0529 cal/gC

5. c = 0.30 cal/gC

6. c = 2.9 cal/gC