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Homework Help: Energy Require from water-steam

  1. Jul 11, 2011 #1
    1. The problem statement, all variables and given/known data
    How much energy is required to change one quarter of 8.0kg of water at 25°C into steam.




    2. Relevant equations
    I get confused to exact what equation to use to figure out the energy required.

    I *think* it would be this one:
    Q[itex]_{t}[/itex]=M[itex]_{water}[/itex]C[itex]_{water}[/itex][itex]\Delta[/itex]T[itex]_{water}[/itex] + .25(M[itex]_{water}[/itex])L[itex]_{f}[/itex]


    3. The attempt at a solution
    When I do the math, I come up with a ridulous number that makes no sense. I try and try to understand but I can't.

    :/.
     
  2. jcsd
  3. Jul 11, 2011 #2

    gneill

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    Staff: Mentor

    What are the values of the constants are you using? What value are you getting?
     
  4. Jul 14, 2011 #3
    Would it be:

    Q=mc[itex]\Delta[/itex]t
    Q=(8.0kg)(4200J/(kg·°C))(100°C-25°C)
    Q=2 520 000

    Then since your finding a a quater of it would you then muliply by .25

    2520000x.25

    =630000J

    It seems to easy since the question is worth 5 marks, it feels like I'm missing a step or something :/
     
  5. Jul 14, 2011 #4

    gneill

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    Staff: Mentor

    It boils down to (pun!) whether you are expected to raise all 8.0 kg of water to 100° and then transform 1/4 of it to steam, or whether you are to take 1/4 of 8.0 kg of water and raise its temperature to 100° and then transform it to steam.

    Your value for Q looks okay for raising the 8.0 kg of water to 100°. How much additional energy will it take to convert 1/4 of that water at 100° to steam at 100°?
     
  6. Jul 14, 2011 #5
    I loved the pun, I actually had a giggle with that one!

    Right now I'll have to find the mass of the steam itself!
    Q=

    To find the additional energy would I use the equation that I initially posted (would I use latent fusion or Latent vaporiztion)?

    BUT I don't know the mass of the steam nor do I know the Energy, so I have NO Idea where to go next :(:(

    (it sucks trying to teach yourself everything from Grade 11 math when you've been out of highschool for 6 years :/)
     
  7. Jul 14, 2011 #6

    gneill

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    Staff: Mentor

    The mass is just 1/4 of 8.0 kg. You want to vaporize this mass of water, so you'd use the latent heat of ______?
     
  8. Jul 14, 2011 #7
    Vaporization since you want to vaporize the mass.

    So with that being said

    Q=(8.0kg)(4200J(kg·°C)(100°C-25°C)+.25(8.0kg)(2300000)
    Q=2520000 + 4600000
    Q=7120000J
     
  9. Jul 14, 2011 #8

    gneill

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    Staff: Mentor

    Looks good!
     
  10. Jul 14, 2011 #9
    Thank you so much!!!
     
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