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Homework Help: Chemistry Calorimetry Question Find Final Temp.

  1. Dec 6, 2011 #1
    1. The problem statement, all variables and given/known data

    10.11g of CaO (molar mass = 56.08 g / mol) is dropped into a styrofoam coffee cup containing 157.0 g of H2O at 25.0ºC. If the following reaction occurs, calculate the final temperature of the water assuming that the cup acts as a perfect insulator.
    (Assume that the solution has the same specific heat capacity as water.)

    CaO(s) + H2O(l) → Ca(OH)2(aq) ∆H = –64.8 kJ

    m = 10.11 g of CaO + 157.0 g of H2O
    ∆T = (Tf - Ti) = (Tf - 25.0ºC
    Cs = 4.184
    q = -64800 J

    2. Relevant equations

    q = mCs∆T

    3. The attempt at a solution

    q = mCs∆T
    -64800 = (167.11g)(4.184)(Tf - 25ºC)
    -64800 = 699Tf - 17480
    -64800 + 17480 = 699Tf
    -47320 = 699 Tf
    Tf = 67.7ºC

    However i know that isn't right,
    The answer i was given was to be 42.8ºC
    So i'm confused as to what i did wrong...
  2. jcsd
  3. Dec 7, 2011 #2


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    Homework Helper

    64.8 kJ heat is released by 1 mol of CaO during the reaction. In the cup, only 10.11 g CaO reacts with water. So how much energy is released?

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