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

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(ΔT), is negative.

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

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

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

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

1. 75 grams fo 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?

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?

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?

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?

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?

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

Tags: Chemistry