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Homework Help: Enthaply of Solution

  1. Feb 10, 2008 #1
    Ok, where am I going wrong on the following problem
    What mass of water is in a solution that decreases in temperature from 25.0 to 20.0 when 2.00 g of NH4Cl are added? (The enthalpy of solution is 14.8 kJ/mol, and the specific heat of the solution is assumed to be 4.18 J/g·K.)

    I set the problem up where the change in temp = q/(specific heat * mass of solution)

    5K = 14800J/(4.18 J/g*K * (2g+xg)) Then I just solved for x & I keep getting around 700g. But the answer is suppose to be 24.5 g. Any ideas where my logic is breaking down?
     
  2. jcsd
  3. Feb 11, 2008 #2
    place a negative sign in front of 5 K -___-;;

    kindly note that change in temperature = final - initial...

    do remember ok? :)
     
  4. Feb 11, 2008 #3
    um... ok, that doesn't really change anything... if you look at my equation I already changed q to a postive 14800, whereas it would have been negative along with the change in temp, its a step I skipped showing. Which still doesn't help me, I'm still getting a little over 700g, & the answer is suppose to be 24.5. If I just change the 5 to a negative, I get negative grams :(. Again, it was already compensated for.
     
  5. Feb 11, 2008 #4

    Bystander

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    How many moles?
     
  6. Feb 11, 2008 #5
    Oh hehheh, bystander spotted it ;p...

    Kuahji, it should be 148000 J/mol rather than 148000 J

    do a conservation of units, you should be able to work out the answer...
     
  7. Feb 11, 2008 #6
    Yes, that work... for whatever reason I kept forgetting J/mol. Thanks again.
     
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