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Homework Help: Stuck on a thermodynamics question (1st law)

  1. Feb 5, 2012 #1
    1. The problem statement, all variables and given/known data

    1.Hibernation is a state of regulated hypothermia, lasting several days or weeks, that allows animals to conserve energy during the winter, During hibernation animals slow their metabolism to a very low level, with body temperature and breathing rates lowered, gradually using up the body fat reserves stored during warmer months. The heat required to sustain animals that hibernate comes from the combustion of fatty acids, one ofwhich is arachidonic acid, a 20 –carbon omega-6 fatty acid: C20H32O2 (s)
    Calculate the mass of arachidonic acid needed to warm a 500.0 kg bear from 5 to 250C
    Assume that the average heat capacity of bear flesh is 4.18 J/g-deg
    ∆H0f (C20H32O2 (s) ) = -636 kJ/mol ; ∆H0f (CO2 (g) )= -393.5 kJ/mol ; ∆H0f(H2O (l) ) = -285.8 kJ/mol

    2. Relevant equations


    Heats of formation

    3. The attempt at a solution

    well the i found the amount of heat lost by the arachidonic acid needed to warm the bear which was -41800 KJ

    and i found the ∆H for the combustion of arachidonic acid using hess' law, which i got -43.3 KJ/mol

    but thing is i have to idea what to do with these two pieces of information, help anyone?
  2. jcsd
  3. Feb 6, 2012 #2
    Hint: If one mole of arachidonic acid produces 43.5 kJ energy, how many moles are required to produce 41800 kJ? Once you have this number, multiply it by the molecular weight of arachidonic acid to get the mass needed.

    I have not checked your calculations. I am assuming they are correct.
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