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Homework Help: Change in entropy problem

  1. May 11, 2010 #1
    1. The problem statement, all variables and given/known data

    0.5 kg of water at a temperature of 5.6 oC is mixed with 1.3 kg of alcohol at 65.9 oC. They come to equilibrium. What is the change in entropy (in J/K) of the system when the two liquids are mixed?

    The specific heat capacity for water is 1 kcal/kgoC = 4186 J/(kgoC), and for alcohol it is 0.58 kcal/kgoC = 2430 J/kgoC.

    2. The attempt at a solution

    I calculate Tf = [m1c1T1+m2c2T2] / [(m1c1) + (m2c2)] = 41.87*C

    then Delta S = m1c1 ln(Tf/T1) + m2c2 ln (Tf/T2) = 2778 J/C

    Then I convert to J/K which is devide it by 274.15 = 10.13 J/K = INCORRECT!!!

    This is a messy problem. Please help!!!
  2. jcsd
  3. May 11, 2010 #2
    Try Cp(water)=4.186 J/g-k and Cp(alcohol)=2.4 J/g-K. Convert temperatures to K and mass to grams. I assume your equations are accurate and the problem is related to units.
  4. May 11, 2010 #3
    Should I take 1 C = 274.15 K or 273 K?
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