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Homework Help: Specific heat of water

  1. Nov 1, 2012 #1
    1. The problem statement, all variables and given/known data

    What is the heat capacity of water in J*g*°C

    2. Relevant equations

    A 74.8 sample of copper at 143.2g
    is added to an insulated vessel containing 165ml of water, Density of water = 1.00 at 25.0 °C.
    The final temperature is 29.7° C
    The specific heat of copper is 0.385 J*g*°C.

    3. The attempt at a solution

    g of water =165g
    quantity of heat= mass of substance * specific heat* temperature change
    heat capacity = C (mass of substance * specific heat)

    quantity of heat of copper = 74.5g*0.3858(29.7-143) =-3.268*10^-3
    so, isnt this the same quantity of heat of water? if so, this how i tried to do it

    165g of h2o* specific heat*(29.7-25)= 3.268*10^-3

    specific heat =4.2147
    so, 165*4.21= 700...
    but thats wrong, the right answer is 74.5g HELP PLEASE!
  2. jcsd
  3. Nov 1, 2012 #2


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    Staff: Mentor

    How come right answer is 74.5 g if the question is about specific heat capacity (which you calculated earlier)?
  4. Nov 1, 2012 #3
    YES! So sorry, I wrote that all wrong and didn't know how to fix it! my units weren't correct.

    It took me a while to see my errors but i managed to solve it. In reality, they were asking for J*mol*°C
    In order to calculate that we just simple do the following
    the total quantity heat of copper which is 74.8*0.385*(143.2-29.7)
    q of copper = -3268.57 -->-3.26*10^-3
    so like no energy can be destroyed or created, the total quantity of heat for water is the opposite which is 3.26*10^-3
    with that being said, now we could calculate the heat capacity and then turn them into moles.
    which is done by the following,
    3268.7 J*g*°C = 165g of H2O * specific heat* (29.7°C-25°C)
    3268.7J*g*°C= Specific heat *775.50

    Specific heat of water = 4.21 J*g*°C , NOW i need to convert that into moles of by multiplying by the molar mass of H2O (18g).

    Final answer is 4.21 * 18 = 75.85 ≈75.9 J*mol*°C

    Thanks for answering anyways!!!
  5. Nov 2, 2012 #4


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    Homework Helper
    Gold Member

    Mynah myah that took me back to school, especially the number which I remembered, let's see that was 4.1 something and then I see 4.12 and it came back. Because that is what used to be called The Mechanical Equivalent of Heat and memory is of countless boring exercises on it converting calories into joules etc. (I think we may even done some of them with non-metric units like food-pounds as well).

    There seemed to be even an ideological aura attached to it because it was about Work not fun or other vanities and came from the no-nonsense working industrial city of Manchester.

    I had not realised till today that Joule's work had generated so much heat: http://en.wikipedia.org/wiki/Mechanical_equivalent_of_heat
    Last edited: Nov 2, 2012
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