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Molar Enthalpy question with Combustion

  1. Jun 27, 2009 #1
    1. The problem statement, all variables and given/known data
    Using the data for the combustion of 3-pentanol calculate the molar enthalpy for the alcohol.

    Initial mass of alcohol: 27.35g
    Final mass of alcohol: 26.41g
    Change in temperature: 21.0C
    Volume of water: 100.0mL (100.0g)
    Specific heat capacity of water: 4.18 J/gC

    2. Relevant equations
    Q=mcΔT

    3. The attempt at a solution
    3-pentanol: C5H12O (88.14818 g/mol)
    27.35g-26.41g=0.94g
    0.94g/88.14818g/mol=0.010663861mol

    Q=mcΔT
    =(100)(4.18)(21.0)
    =8778J
    ΔH=-8778J
    ΔH/n=-8778J/0.0106638161mol
    =-823154.0152J/mol
    =-8.23x10^2kJ/mol ←is this the correct answer? (I don't have an answer key.)
     
  2. jcsd
  3. Jun 29, 2009 #2

    chemisttree

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    It looks good. The only complicating factor is to determine if the 3-pentanol underwent an increase in temperature as well. You used about a gram of the alcohol and about 26 grams are left. Did that remaining alcohol undergo an increase in temperature of 21oK?
     
  4. Jun 29, 2009 #3
    I believe the remaining alcohol increased 21 Celcius.
    I just want to know if I determined the molar enthalpy of alcohol in the end.
     
  5. Jun 29, 2009 #4

    chemisttree

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    If that is the case you will need to revise (very slightly) your answer to account for the heat required to heat 26.41 g of the alcohol through 21o. It's probably small enough to neglect.
     
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