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Homework Help: Quantitive Energy problems

  1. Nov 10, 2008 #1
    1. The problem statement, all variables and given/known data
    a cup of coffee (140g) cools from 75 degrees C down to 20. degrees C. how much energy does it release to the surroundings? answer in J

    suppose dutring volleyball practice, you lost 2.0 lbs of water due to sweating. if all of this water evaporated, how much energy did the water absorb from your body? answer in kJ.

    suppose during a lab that 65kJ of energy was transferred to 450 g of water at 20C. what would be the final temperature of the water? answer in C

    the heat capacity of solid iron is 0.447 J/g degrees C. If 65kJ was transfered to a 450 g chunk of ioron at 20. C what would be the final temperature?

    a serving of cheezits releases 130kcal when digested by your body. if this same amount of energy was transferred to 2.5kg of water at 27 C what would the final temperature be?

    if 130kcal was transferred to 2.5kg of water at its boiling point, what fraction of the water would be vaporized?

    suppose a bag full of ice (450g) at 0.0 C sits on the counter and begins to melt to a liquid state. how much energy must be absorbed by the ice if 2/3 of it melted?

    2. Relevant equations

    Energy constants
    334 J/g heat of fusion
    2260 J/g heat of vaporization
    2.1 J/g heat capacity of solid water
    4.18 J/g heat capacity of liquid water

    3. The attempt at a solution If you could show work with answer that would be very apreciated as i wasnt there the day when he lectured us on how to do this. an explanation of the problems would be awesome too!

    i assume that you multiply the mass times the heat capacity and that all equals the energy (Q). but i dont know what to do then... there is something im missing
    Last edited: Nov 10, 2008
  2. jcsd
  3. Nov 10, 2008 #2


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    Staff: Mentor

    These are all about heat balance. Heat gained = heat lost (by different parts of the system). Heat = m*c*delta T.
  4. Nov 10, 2008 #3
    delta T?
  5. Nov 11, 2008 #4


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    Staff: Mentor

    Change in temperature. Like in

    delta T = Tfinal - Tinitial = 75 - 20 = 55 deg C
  6. Nov 11, 2008 #5
    oh ok ty
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