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Homework Help: Temperature Problems Confused please help!

  1. Apr 22, 2005 #1
    Temperature Problems... Confused!! please help!

    How much heat must be taken away from 400 grams of steam at 200 degrees to change it into water at 100 degrees? (Teacher says answer is -985,000 J)

    How much heat must be absorbed by 1 kg of ice at -20 degrees Celcius to change it to steam at 130 degrees? (Answer is 3,100,000 J)

    If someone could please show me how to work out these problems, it would be wonderful!!!!! Thank you so much!!! :smile:
  2. jcsd
  3. Apr 22, 2005 #2

    Andrew Mason

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    How many moles of H2O in 400 g. of steam? What heat is given off in cooling to 100 deg. C? What is the heat removed from water turning from vapour to liquid?

    Use the same approach as above.

  4. Apr 24, 2005 #3
    its very very easy......

    what andrew said is right...but i will explain it to u......
    when steam is to be converted into water it has to lose heat.....but one important thing u have to notice is that...when a substance is changing its phase its losing more heat than when a small temperature change occurs.....
    first lets take steam from 200 to equilibrium..ie 100 degrees......for that heat required is equal to....Q=.4*c*(373-473)........now,steam has to spend some energy to convert steam into water.....for that Q1=L*.4...where L is the latent heat of vapourization...........then add Q & Q1......remember the added value will be negative as the heat energy is lost here and not gained.....remember to convert grams into kilograms for getting answer in SI units...........do the same for the second question as well....here Q maybe positive as here we need more energy to convert a solid to vapour.......

    finally just remember this.......we need a lot of energy to change the phase of a substance......and always calculate Q by dividing it into various Qs wherever a phase change is taking place....ok yaaaaaar
    Last edited: Apr 24, 2005
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