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Homework Help: Enthalpy Change of Combustion.

  1. Jul 11, 2012 #1
    1. The problem statement, all variables and given/known data
    When 1.00g of ethanol was burned under a container of water, it was found that 100g of water was heated from, 15°C to 65°C, The process was known to be only 70% efficient. Calculate the enthalpy change of combustion of ethanol taking specific heat capacity of water to be 4.18 J g^-1 k^-1.

    2. Relevant equations
    q= mc ΔT; ΔHr= - q/ moles of limiting reagent.

    3. The attempt at a solution
    I calculate q = 100 x 4.18 x 50 x(70/100) = 14630J.
    Then I found the number of moles of ethanol which is 1/46 = 0.0217
    Therefore enthalpy change will then be -14630/0.0217 = -672980J = -673 kJ mol^-1 but the answer given is -1.37 x 10^3 kJ mol^-1. Where did I go wrong??
  2. jcsd
  3. Jul 11, 2012 #2
    In your first step you should be dividing by 70% rather than multiplying by 70%.
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