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Homework Help: Specific/Latent Heat and Melting Iron!

  1. Jun 25, 2008 #1
    1. The problem statement, all variables and given/known data
    How much heat is needed to melt 16.7 kg of iron that is initially at 21.3°C?

    2. Relevant equations
    Q=m*c*ΔT + mL

    3. The attempt at a solution
    here: m=16.7kg, c=0.45kJ/kg*K, and ΔT=1538-21.3=1516.7

    Thus, Q1=1.14e7 J

    Then, I need to calculate latent heat needed, which is Q2 = (2.67 x 10^5)* (16.7) = 0.44584 x 10^7

    So, the total heat needed is: Q1+Q2=1.55x10^7 J. I also got 1.58x10^7 on my second try.

    But neither are the right answer. What did I do wrong?
  2. jcsd
  3. Jun 25, 2008 #2


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    Homework Helper

    I think Q1 is 1.14e4 J (according to my calculator).

    But I found like 3 values for the latent heat of iron

    Iron, gray cast 96 kJ/kg
    Iron, white cast 138 kJ/kg
    Iron, slag 209 kJ/kg

    So not too sure which one is correct for the question.
  4. Jun 25, 2008 #3


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    Staff: Mentor

    J or kJ?
  5. Jun 25, 2008 #4


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    Homework Helper

    ah didn't notice the units of c...but I'd still get the OP's answer for the 2nd try using that value of the latent heat.
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