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Absorption Of Heat; water and steam thermodynamics

  1. May 24, 2009 #1

    K29

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    1. The problem statement, all variables and given/known data
    Steam initially at 120 oC is passed into 0.60 kg of water initially at 20 o C, and the total mass of water is just brought to the boil at 100C

    What is the mass of steam that has condensed? Ignore the thermal capacity of the container and heat losses.



    2. Relevant equations

    Q = cm(T2-T1) heat = specific heat * mass * change in temperature
    Q = Lm heat = heat of Vaporisation or Fusion * mass
    Heat of vaporisation of water is 2260 kJ/kg
    Specific heat of water is 4200 J/kg.oC
    Specific heat of steam is 2100 J/kg.oC


    3. The attempt at a solution
    Qgained by water being heated = Qlost by steam being cooled + Q Lost by steam being condensed

    cmw(T2-T1) = cmstot(T2-T1) +Lmscondensed

    Unfortunately I can't seem to get rid of mstot. It is not given and I need to either work it out or find another equation. Help!
    If it helps, the final answer should be 87g
     
  2. jcsd
  3. May 24, 2009 #2

    Mapes

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    If the problem can't be solved otherwise, why not assume that all the steam condensed?
     
  4. May 24, 2009 #3

    K29

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    I have tried this and get
    |cmw(T2-T1)| = |cms(T2-T1)| +|Lms|
    |4200 * 0.6 (100-20)| = |2100 * ms(100-120)| + |2260ms|
    |201600|= |-42000ms|+|2260ms|
    201600 = 42000ms+2260ms
    201600=44260ms
    ms=4.55 kg

    As I said above the answer given in the notes is 87g = 0.087kg :(
     
  5. May 24, 2009 #4

    Mapes

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    Check your units.
     
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