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    I came across this OCR question, and don't understand part iii.

    Why would you not divide the energy of 1 mole of particles by the temperature and the mass of 1 mole since e = mcΔT, so
    c = e / mΔT ?

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    i assume the answer to part (i) gave you the total energy?
    the reason you can't do that is because there is not a change in temperature and the energy in mcdT is only for a change of temp with no change in state. Also you are not trying to work out c (the specific heat capacity) but the average energy of each atom in the aluminium.

    So if you know the total energy you just need to divide by the number of atoms, and to do that you use the moles calculations.
    Mass/Mr=mols
    1 mol = 6.02x10^23atoms
    then divide total energy by number of atoms to get the average energy.

    I hope that helps. Let me know if i can clarify it any further.
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    (Original post by macman83)
    i assume the answer to part (i) gave you the total energy?
    the reason you can't do that is because there is not a change in temperature and the energy in mcdT is only for a change of temp with no change in state. Also you are not trying to work out c (the specific heat capacity) but the average energy of each atom in the aluminium.

    So if you know the total energy you just need to divide by the number of atoms, and to do that you use the moles calculations.
    Mass/Mr=mols
    1 mol = 6.02x10^23atoms
    then divide total energy by number of atoms to get the average energy.

    I hope that helps. Let me know if i can clarify it any further.
    I think you've explained part ii? I managed to work that bit out, its part iii that I dont seem to understand
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    (Original post by voltz)
    I came across this OCR question, and don't understand part iii.

    Why would you not divide the energy of 1 mole of particles by the temperature and the mass of 1 mole since e = mcΔT, so
    c = e / mΔT ?

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    That would appear to get you an answer of 24.7 as well (after converting deg C to K)

    The markscheme shows an example of the working - it's not supposed to be an exhaustive list of all possible acceptable methods - the question leaves it to you to come up with a valid method and if you do that you should get the marks.
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    (Original post by Joinedup)
    That would appear to get you an answer of 24.7 as well (after converting deg C to K)

    The markscheme shows an example of the working - it's not supposed to be an exhaustive list of all possible acceptable methods - the question leaves it to you to come up with a valid method and if you do that you should get the marks.
    I can't seem to get that answer. This is the calculation I am doing:

    Total energy of 1 mole = 1.2x10^-20 * 6.02x10^23
    Then I used e = mcdT, with 0.027 as the mass and 293 as the temperature. I get an answer around 920 which is just the specific heat capacity given. I dont seem to understand why you wouldn't need to divide by the mass of 1 mole
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    (Original post by voltz)
    I can't seem to get that answer. This is the calculation I am doing:

    Total energy of 1 mole = 1.2x10^-20 * 6.02x10^23
    Then I used e = mcdT, with 0.027 as the mass and 293 as the temperature. I get an answer around 920 which is just the specific heat capacity given. I dont seem to understand why you wouldn't need to divide by the mass of 1 mole
    it's asking for the molar heat capacity - the amount of heat per mole per Kelvin


    as you said

    Total heat per mole (at 293K... 20 deg C) = 1.2x10^-20 * 6.02x10^23
    which is equal to 7224 J

    7223/293=24.7
    you're literally dividing the heat per mole by the number of kelvin to get the heat per mole per kelvin the question asks for.

    I don't see why you think you want to bring the mass into it.
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    (Original post by Joinedup)
    it's asking for the molar heat capacity - the amount of heat per mole per Kelvin


    as you said

    Total heat per mole (at 293K... 20 deg C) = 1.2x10^-20 * 6.02x10^23
    which is equal to 7224 J

    7223/293=24.7
    you're literally dividing the heat per mole by the number of kelvin to get the heat per mole per kelvin the question asks for.

    I don't see why you think you want to bring the mass into it.
    I simply tried plugging the values in to e=mcdT so and the mass from that equation is where im getting it from
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    (Original post by voltz)
    I simply tried plugging the values in to e=mcdT so and the mass from that equation is where im getting it from
    The question's asking for the heat capacity of one mole of aluminium... i.e. the heat capacity of a piece of aluminium with the mass of one mole of aluminium atoms.

    you don't want to convert the mass of one mole of aluminum into kg and give the heat capacity of 1kg of aluminium instead, because it's not what the question is asking for.
 
 
 
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