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Homework Help: AP Chem; Calculating energy released

  1. Sep 6, 2007 #1
    I have just started AP Chemistry this year as a junior at my highschool despite all peer protests against it. And I admit so far the course has lived up to it's reputation of being very difficult. I was rather ecstatic to come across this site where I may obtain the help I /know/ I will need all year. So expect to see me often. ^_^
    We use the 7th edition Chemistry textbook by Zumdahl, so I believe my source is familiar to at least some.

    1. The problem statement, all variables and given/known data

    Calculate the energy released per (4 mass #, 2 Protons)He nucleus produced and per moles of (4 mass #, 2 Protons)He produced. The atomic masses are (2,1)H, 2.01410; (3,1)H, 3.01605; and (4,2)He, 4.00260. The masses of the electron and neutron are 5.4858 x 10 raised to the -4 and 1.00866 amu, respectively.

    2. Relevant equations

    The easiest fusion reaction to initiate is

    (2,1) H + (3,1) H ---> (4,2) He + (0,1) n

    3. The attempt at a solution

    ∆m = (4.00260 + 1.00866) - (2.01410 + 3.01605) = -1.02755 amu
    (Originally I had the two enclosed equations swapped which would give a positive result instead. Would this matter? If so, why?)

    For Mole;
    -1.02755 amu = 1.6005 x 10 to the -27 Kg / 1 amu
    = -1.6446 Kg

    E= mc squared, so: -1.6446 Kg x (2.997 x 10 to the 8) Squared
    = 49289 J

    For Nucleus;
    I was unsure how to find this, but I found that
    1 mol = 6.0221 x 10 to the 23 nuclei
    So do I need to convert Kg into mol and then into nuclei?
  2. jcsd
  3. Sep 7, 2007 #2


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    You can express the energy in terms of MeV. 931.5 MeV/amu is the conversion factor I believe. To convert that to joules, use the equality 1 electron volt = 1.602 X 10^-19 Joule and MeV is million electron volts. The question asks you to give the energy in terms of 'per He nucleus' and in terms of 'per mole of He nucleus'. Find the energy per nucleus of He and then multiply that by Avogadro's number to get it in terms of energy per mole.
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