1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

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


    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    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.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook