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Find the latent heat of vaporization

  1. Jan 27, 2008 #1
    Simple Q - Latent Heat of vaporization H20 @15 C and 115 C

    sorry , I have reposted in the homework help section...

    I am not good a math and physics and I am taking a intro Food Engineering course. My question on my assignment states " Find the latent heat of vaporization for water at 15 °C and 115 °C"

    I know it is very simple but it seems like there is info left out for us to assume? or perhaps this is just found looking in tables. Im just not sure the correct answer.

    Thanks for any help!

    Mibson
     
    Last edited: Jan 27, 2008
  2. jcsd
  3. Jan 27, 2008 #2
    1. The problem statement, all variables and given/known data

    I am not good a math and physics and I am taking a intro Food Engineering course. My question on my assignment states " Find the latent heat of vaporization for water at 15 °C and 115 °C"

    I know it is very simple but it seems like there is info left out for us to assume? or perhaps this is just found looking in tables. Im just not sure the correct answer. I have search online and keep finding the equation Q = mL

    Thanks for any help!

    Mibson

    2. Relevant equations

    I know the equations in my notes but I dont think this even requires a calculation.

    The molecular formula of water is H20 so perhaps I need the molecular mass of H and O thus 1.00794 and 2x(15.9994) = 33.00674 ; is this g/mol (its been 10 years since I took chemistry)

    Latent Heat of Vaporization = Hg - Hf = Hfg (this is in my notes)

    What I dont understand is that I thought the Latent Heat of Vaporization is for when a phase change is happening and I thought phase changes occur at ~0C and ~100C

    Also in my notes it says Hg - Hf = Hfg = 2373.06 kJ/kg,


    3. The attempt at a solution

    I have read my notes 4 times and been doing this simple question for over two hours. It seems to me like the prof has not stated things we must assume to answer the question.

    Perhaps it is just 2272 J/g?
     
    Last edited: Jan 27, 2008
  4. Jan 27, 2008 #3
    Well there really isn't much to assume. Things you know is the molecular formula of water, and you can easily find it's specific heat & heat of vaporization.

    What formula do you think you will need to use in order to calculate it's latent heat of vaporization?
     
    Last edited: Jan 27, 2008
  5. Jan 27, 2008 #4
    The molecular formula of water is H20 so perhaps I need the molecular mass of H and O thus 1.00794 and 2x(15.9994) = 33.00674 ; is this g/mol (its been 10 years since I took chemistry)

    Latent Heat of Vaporization = Hg - Hf = Hfg (this is in my notes)

    What I dont understand is that I thought the Latent Heat of Vaporization is for when a phase change is happening and I thought phase changes occur at ~0C and ~100C

    Also in my notes it says Hg - Hf = Hfg = 2373.06 kJ/kg,

    Thanks for the help!
     
    Last edited: Jan 27, 2008
  6. Jan 27, 2008 #5

    russ_watters

    User Avatar

    Staff: Mentor

    It sounds like this may be something you need a steam table for (hg, hf, and hfg are out of a steam table). Do you have one?

    IIRC, you can also use Hfg and add to it the h required to change the temperature from 15->100C (for the first part...). This would be based on Cp * dT (specific heat times temperature difference).

    At some temperatures and pressures, water can exist in more than one phase. If it is evaporationg, it is changing from a liquid to a gas, and as you know air at normal everyday temperatures is capable of holding a certain amount of water vapor.
     
  7. Jan 28, 2008 #6
    I have a steam table but dont undertand it completely. I have to chose one of two tabs : staturated OR superheated/subcooled ; The only one where temperature is the only input is the saturated one but I dont see how there would only be steam at 15C perhaps at 115C it will all be steam. Basically using the "staturated" tab does not make alot of sence to me as I dont see how the h2o at 15C would be saturated steam...
     
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