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Homework Help: HELP w/ Thermodynamics Problems?

  1. Mar 24, 2010 #1
    HELP w/ Thermodynamics Problems???

    i am absolutely dying with these probs. been working on them for hours. due tomorrow, so any help greatly appreciated...thanks so much.

    1. The problem statement, all variables and given/known data
    1. A 4.00 102 g glass coffee cup, m, is 20.0°C, ti, at room temperature. It is then plunged into hot dishwater at a temperature of 77.0°C, tf, as shown in Figure 12-18. If the temperature of the cup reaches that of the dishwater, how much heat does the cup absorb? Assume that the mass of the dishwater is large enough so that its temperature doesn't change appreciably.

    2. A 12.0 kg piece of zinc, m1, at 71.0°C is placed in a container of water, as shown in Figure 12-19. The water has a mass of 18.0 kg, m2, and has a temperature of 10.0°C before the zinc is added. What is the final temperature of the water and zinc?

    3. The kinetic energy of a compact car moving at 100 km/h is 2.9 * 10^5 J. To get a feeling for the amount of energy needed to heat water, what volume of water (in liters) would 2.9 105 J of energy warm from room temperature (16°C) to boiling (100°C)?

    4. How much heat is needed to change 5.00 * 10^2 g of ice at -20.0°C to steam at 195.0°C?

    2. Relevant equations
    Q = mcAt
    Q = -Q (transfer of energy/heat)
    K = C + 273
    anything thermo related basically...

    3. The attempt at a solution
    1. i just keep getting the wrong answer (this is a webassign). i assume you solve for a Q on one side of the equation, and that would be the answer? i don't know. i'm totally lost and confused. not to mention my teacher never gave me the specific heat capacity for glass :[

    2. i did mcAt = mcAt, and got 5.96 C....wrong answer. did the prob about 5 more times. different answers. all wrong. failure. help needed. please.

    3. 2.9 *10^5 J=m(4.18)(change in temp)..i got 824.55....wrong.

    4. ok, so this is the one i've done approximately 12 times and still keep getting wrong. i know the process...you solve it by intervals, using mcAt for the rising slopes and heat of fusion/vaporization for the plateaus (in the phase change graphs).


    you have to do it for each increment

    -whatever to 0
    the phase change
    0 to 100
    the second phase change
    100 to whatever"

    ...BUT...i keep getting the wrong answer. 2427730350 <--the one i just got....which is WRONG.

    please please please help....i'm really about ready to shoot myself. i'm not a math person. at all. and i really can't fail this class. :[
    Last edited: Mar 24, 2010
  2. jcsd
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