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Homework Help: Molar enthalpy heat of vaporization

  1. May 14, 2007 #1
    1. The problem statement, all variables and given/known data
    Ethanol boils at a temp. of 78.29 degrees C. What amount of energy, in joules, is necessary to heat to boiling and then completely vaporize a 23 g sample of ethanol initially at 12 degrees C?

    2. Relevant equations
    q=mass x constant x change in temp ????

    3. The attempt at a solution
    I know that the molar enthalpy heat of vaporization is 35.56 kJ/mol and the specific heat of ethanol (C2H5OH) is approximately constant at 2.44 J/gK. However, I am unsure of what to do with these numbers or even which equation to use. Any help at all will be appreciated!
  2. jcsd
  3. May 14, 2007 #2


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    Homework Helper

    You can consider this in two parts: the first part is the thermal energy needed to bring the ethanol from 12 degrees to its boiling point. You can use the equation you posted for this. Then you must consider the heat of vaporization when the ethanol undergoes the phase change to the gas state. You should be able to find what you need to do this in your textbook. Give that a try.

    Caution: be careful of your units when doing the calculations, since you have quantities expressed in both grams and moles.
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