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Thermodynamics Help

  1. Jan 16, 2008 #1
    1. The problem statement, all variables and given/known data
    How much heat is required to raise the temperature of 0.250kg of water from 20.0oC to 30.0oC?

    If this amount of heat is added to an equal mass of mercury that is initially at 20.0oC, what is its final temperature?


    2. Relevant equations

    Q=c*M*(t2-t1)

    3. The attempt at a solution

    for the first part i got
    q=1*0.250*(30.0-20.0)
    q=10.4725
    this is wrong the correct answer was 1.05*10^4
    how did they get that answer?

    The second part i have so far
    1.05*10^4=c*0.250*(t2-20)
    it didnt say what the spefice heat of mercury was so i looked it up and it was 0.140
    1.05*10^4=0.140*0.250*(t2-20)
    1.05*10^4/(.035)+20

    please help with this
     
  2. jcsd
  3. Jan 16, 2008 #2

    hage567

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    For the first part why are you using 1 for c? Look up the specific heat of water. Regardless, I don't see how you got 10.4725 from your equation for q. Is there a typo in there somewhere?
     
  4. Jan 17, 2008 #3

    Andrew Mason

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    You have to be clear on your units. You appear to be working in calories and kg. If so, you have to use a specific heat for water in calories/kg deg not calories/g deg.

    AM
     
  5. Jan 17, 2008 #4
    i just said find the answer in joules
     
  6. Jan 17, 2008 #5

    Andrew Mason

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    If you are using a specific heat of 1 for water you are working in calories/g. Kelvin. If you want the answer in joules you have to multiply the answer by 4.186 (ie. 4.186 J in one cal.). The specific heat for water in Joules/Kg K is 4186.

    AM
     
    Last edited: Jan 17, 2008
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