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!

Calculating Final Temperature

  1. Nov 18, 2009 #1
    1. The problem statement, all variables and given/known data

    A piece of iron weighed at 120 grams at 95.0 degrees Celcius is added to 750ml water of 25.0 degrees Celcius. What is the final temperature of the water when it is equilibrated? Assume the dentisy of water is 1 g/ml. (Specific heat of water is 4.184 and specific heat of iron is .449).

    2. Relevant equations

    q=mc(delta t). In this case the q of iron is equal to the q of water.

    3. The attempt at a solution

    Here is my setup:



    I know this is wrong, but I'm not sure if it's because I set this up wrong or did my calculations incorrectly.
    Last edited: Nov 18, 2009
  2. jcsd
  3. Nov 18, 2009 #2


    User Avatar
    Science Advisor
    Homework Helper

    It's a sign error, the change in temperature from 95>x and 25->x must be different signs
  4. Nov 18, 2009 #3
    I don't understand what you mean. I know that 23.85 implies that the temperature decreased for both the iron and water, and that is wrong.
  5. Nov 19, 2009 #4


    User Avatar

    Staff: Mentor

    mgb points out that there is a sign error in your original equation.

  6. Nov 19, 2009 #5
    I think I forgot the qin=-qout concept. Let me try again.


    x=26.18 degrees Celcius

    Is this what you were referring to?
  7. Nov 19, 2009 #6


    User Avatar
    Science Advisor
    Homework Helper

    if one thing is cooling from 95deg to x then the change in T is (95-x)
    then if the other is heating form 25 to X the change must be (x-25) which gives the different signs.

    Putting a minus on one side is the same thing (or at least has the same effect)
  8. Nov 19, 2009 #7
    I thought that in order to do the equation correctly, one must always subtract the initial temperature from the final temperature.
  9. Nov 19, 2009 #8


    User Avatar
    Science Advisor
    Homework Helper

    The change in temperature defines the heat flow, so heat in and heat out would have different signs.
  10. Nov 19, 2009 #9
    I see. Thank you for checking out my question and taking the time to explain it. :)
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook