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Chemistry question

  1. Oct 26, 2005 #1

    DB

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    can someone give me a hand on this one?

    How many grams of water at 20 degrees Celsius can be converted into steam at 500 degrees Celsius with 10 kJ of energy?

    wat i know is that this question is 2 parts: Water being heated to 100 C (80 C change)using the specific heat of water n then having a temperature change of 400 C using the specific heat of steam.

    wat i dont understand is how i am supposed to split up the 10 kJ? do i have to have only 1 equation?

    thanks in advance
     
  2. jcsd
  3. Oct 26, 2005 #2

    DB

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    sorry guys, i just realized there was a chemistry section, my bad...
     
  4. Oct 26, 2005 #3

    Doc Al

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    Staff: Mentor

    There are three steps: (1) Heating the water, (2) converting water to steam, (3) Heating the steam.

    Express the heat required for each step in terms of the mass, m. Set the total energy equal to 10 kJ. That's the equation you want.
     
  5. Oct 26, 2005 #4

    DB

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    okay but should i have 3 equations or 1?
     
  6. Oct 26, 2005 #5

    Doc Al

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    Staff: Mentor

    You'll end up with a single equation. (Reread my previous post.)
     
  7. Oct 26, 2005 #6

    DB

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    i dont no wat u mean when u say express the heat in terms of mass
     
  8. Oct 26, 2005 #7

    DB

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    wat i tried was adding each step and then dividing 10000J by the sum to solve for m, i got 2.94 grams....?
     
  9. Oct 26, 2005 #8

    Doc Al

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    Staff: Mentor

    For example: For step 1, how much heat is required to heat "m" grams of water from 20 degrees to 100 degrees?
     
  10. Oct 26, 2005 #9

    DB

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    okay so now i've got 3 values for "m":

    [tex]m=\frac{E}{80*4.19}[/tex]

    [tex]m=\frac{E}{2262}[/tex]

    [tex]m=\frac{E}{400*2.01}[/tex]

    now im stuck :(
     
  11. Oct 26, 2005 #10

    Doc Al

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    Staff: Mentor

    There is only one "m". What you should have is three values for E: [tex]E_1, E_2, E_3[/tex]. Your equation would be: [tex] E_1 + E_2 + E_3[/tex] = 10 kJ; solve for m.
     
  12. Oct 26, 2005 #11

    DB

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    ya thats wat i did when i said i got 2.94 grams
     
  13. Oct 27, 2005 #12

    DB

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    anyway im convinced that the answer is 2.94 grams, thanks for the tips doc al, i apreciate it
     
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