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Homework Help: Specific Heat Problem (did something wrong?)

  1. Nov 30, 2017 #1
    1. The problem statement, all variables and given/known data
    Heated iron with mass of 55.0 g was added to 100 mL of water at 20 degree Celsius. Assuming no energy transfer to the surroundings and that the final temperature of the system is 42.7 degrees C, calculate initial temperature of the iron.

    Mass of Iron = 55.0 g
    mass of Water = 100g
    initial Temp of Water = 20 degrees C
    Final temp of Water/Fe = 42.7 degrees
    Want: initial temp of Fe

    Given Cs of Fe = 0.451 J/gC
    Cs of Water = 4.184 J/gC
    2. Relevant equations

    heat of Iron = -heat of water (q=-q)

    mCT = mCT
    3. The attempt at a solution
    mCT of Fe = -mCT of water

    plug in:
    X= initial temp of Fe
    55 * 0.451*(42.7-X) = 100*4.184*(42.7-20)

    When i solve for X , i get a large negative number = -9497.68 which is clearly not the final temperature!

    Was wondering where i went wrong!
  2. jcsd
  3. Nov 30, 2017 #2
    It looks like you lost the negative sign. It should be
    $$55 \cdot 0.451\cdot(42.7-X) = -100\cdot 4.184\cdot (42.7-20)$$
  4. Dec 1, 2017 #3
    Wow, didn't realize that. Thanks !
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