Chemistry(Please help, thank you)

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For the equation:

deltaHrxn = qrxn / mol substance to calculate deltaH per mole

what does the mol substance mean? Does this mean that you use the mm of the substance or if you already calculated the # of moles of a substance do you use that?

  • Chemistry(Please help, thank you) -

    It means you use the #moles of the substance you've already calculated.

  • Chemistry(Please help, thank you) -

    ok thank you very much!!!

  • Chemistry(Please help, thank you) -

    My qrxn value was 30267.23J and # of moles was 6.25e-3mol so that = 4842756.8 but are the units J/mol and do you think I should convert to kj?

  • Chemistry(Please help, thank you) -

    If you used specific heat as 4.184 J/g*C then the units you have are J/mol. Most tests quote kJ/mol for this type of work.

  • Chemistry(Please help, thank you) -

    The units for the specific heat of water that they gave me were J/ g x K

  • Chemistry(Please help, thank you) -

    So in that case I would just divide my answer of4842756.8 by 1000 = 4842.76 kJ/mol correct?

  • Chemistry(Please help, thank you) -

    yes.

  • Chemistry(Please help, thank you) -

    I now need to find the enthalpies for Mg and MgO and then use the equations
    Mg(s) + 2HCl(aq) -> MgCl2(aq) + H2(g)
    MgO(s) + 2HCl(aq) ->MgCl2(aq) + H2O(l)
    H2(g) + 1/2O2(l) delta H =-241.8 kJ

    I do not understand how you find the values for the first two equations when there is only one value given.

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