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Heat capacity problem-wher am i going wrong?

  1. Dec 2, 2006 #1
    1. The problem statement, all variables and given/known data
    Hi,so iv tried the following question and think my general methodology is correct but the final answer i am getting is wrong.ne ideas?

    2. Relevant equations
    Question:a styrofoam cup contains 200g of water at 20 degrees celsius.What will be the equilibrium temp of the system after 100g of silver,initially at 300 degrees,has been added?you may assume that the heat capacity of the cup is negligible and that no heat is lost to the surroundings.

    (b)a further 100g of an unknown substance,also at 300 degrees is now added to the system and the temperature rises by a further 26 degrees celsius
    what is the specific heat capacity of the unknown substance?(the specific heat of water is 4180 Jkg^-1 K^-1 and that of silver 240 J kg^-1 K^-1)

    3. The attempt at a solution

    so i got part a to work out correctly by saying:
    heat energy lost by silver = heat energy gained by water
    mass(silver).c(silver).(initial temp-final temp) =mass(water).c(water).(final temp-initial temp)

    i.e. 0.1(240)(300-final temp)=0.2(4180)(final temp-20)
    and found the equilibrium temp=28 degrees celsius,which i know is correct

    its part b im having trouble with:the correct answer is supposed to be 910 J kg^-1 K-1 but i keep getting 8600,by using the following,is it correct:

    heat energy lost by unknown substance= heat energy gained by water and heat energy gained by silver

    mass(unknown).C(unknown).(26)=mass(water).c(water).(26) + mass(silver).c(silver).(26)

    i.e. 0.1(c)(26) = 0.2(4180)(26)+0.1(240)(26)

    => c= 8600..which is wrong according to book?
    any help wud be much appreciated
    thanks :biggrin:
  2. jcsd
  3. Dec 2, 2006 #2


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    What is the temperature change for the unknown mass?

    [What is the final temperature for all three? = 28 + 26]

    Using this, and your method, I get 908.9, which is close enough for me.
    Last edited: Dec 2, 2006
  4. Dec 3, 2006 #3
    ok ya that seems to make alot more sense alright
    thanks for the help:tongue:
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