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Homework Help: Determine the heat gained by the water

  1. Feb 17, 2010 #1
    1. The problem statement, all variables and given/known data
    I'm having a problem in solving specific heat equations. I get the basic concept and know the formula, but in doing the sample problems, I can't seem to get them right. I have a lab assignment where I need to find the specific heat capacity for Fe. I have all the required information filled in on the observation sheet.

    Here's the problem:
    Mass of Fe: 100gm
    Volume of water in calorimeter:
    Mass of water in calorimeter:
    Initial Temp:20 degrees C
    Final Temp: 100 degrees C
    Final Temp of both the metal and water: 100 degrees C

    Determine the heat gained by the water (show your work)
    Q water=
    Q Fe=
    2. Relevant equations

    Q= C(specific heat) x Mass x change of temperature

    3. The attempt at a solution

    What I have done so far is
    Mass= 100
    C= 0.108
    Tf= 100 C
    TI=20 c

    from there Q=100*0.108*80= 864 (I know somethings wrong with that answer)
    I think it has to do with C, I tried finding the Specific heat of Iron online and came up with several different answers
  2. jcsd
  3. Feb 20, 2010 #2
    Its not working because this is a two part problem. The specific heat of water is a very commonly known number and you should have it memorized (I know I had to for my class). I believe it is something like 4.18J/g*K. So you need to set up the equation to get the change in enthalpy for water then use that number to set up a new equation where the change in enthalpy is the same magnitude but opposite direction for the iron and solve for the specific heat of iron.

    Remember that the conceptual understanding is even more important than doing the actual math in these types of problems. You have to be able to understand how the system and environment interact, where the heat flows, what numbers will they have in common (remember energy is always conserved so if I lose X amount of heat something somewhere has to gain that exact same amount of heat). You need the initial temp of the water, the final temp of the water, the mass of the water (you can find this if you know the volume by using the density formula). All of those numbers plus the commonly known specific heat of water will allow for calculation of change in enthalpy for the water. The change in enthalpy will be the same magnitude but opposite direction for the iron so you can plug in the numbers (this is where you use the 100g and whatever the delta T for the iron was) and solve for the Cs.
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