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Homework Help: Chemistry homework help with enthalpy

  1. Oct 8, 2014 #1
    1. The problem statement, all variables and given/known data

    A 34.05 gram liquid sample of an unstable compound is deposited in a metal piston assembly that has a cylinder with a diameter of 20.0 cm. After the sample is added, the piston is placed in contact with the liquid and the piston vent is closed. This process removes all residual gas from the piston. The piston assembly is placed in 15.00 Kg of water at 30.00 oC. The barometric pressure is 778 mm Hg (760 mm Hg= 1 atm). As the compound spontaneously decomposes the piston moves up and the temperature of the water increases to 34.52oC. The piston rises 59.1 cm during the reaction. The contents of the cylinder at the end of the complete decomposition consists of 0.450 mol of carbon dioxide gas,0 .375 mol of liquid water, 0.0375 mol of oxygen gas and an unknown amount of gas from an unidentified element. Other useful information:

    ∆Hfo CO2 = -393.5 Kj/mol
    ∆Hfo H2O = -286 kj/mol
    Enthalpy of decomposition for unknown = -1893Kj/mol
    Heat capacity of water = 4.184 J/goC
    R = 0.08206 L x atm / mol x K or 8.3145 J/mol x K
    Vapor pressure of H2O at 34.5oC = 41 mm Hg

    Assume that the heat capacity of the piston assembly is negligible and the mass of the piston is negligible.

    1- The formula of the unstable compound.
    2- The pressure –volume work in kJ for the decomposition of the 34.05 gram sample.
    3- The molar change in internal energy for the decomposition of the compound.
    4- The standard enthalpy of formation for X

    2. Relevant equations
    The only relevant one that I can think of so far is the q equation which is q = total mass * heat capacity * change in temp

    3. The attempt at a solution

    I don't expect you guys to give me the answer to the whole sheet, but if you could help point me in the right direction for each of the problems then I would be very appreciative.

    The only thing I know so far is to find the change in heat which would be:
    q = 15034.05 * 4.184 * -4.52 = -284319.14 J = -284.319 KJ.

    Since it says the enthalpy of decomp for the unknown is -1893 kj/mol I think that we would use the -284.319 kj to figure out the moles of the unknown that we have. This would be around .15 moles of the unknown. If .15 mol = 34.05g then 1 mol = 227 grams/mol. Is this correct?

    I am not sure where to go from here at all.
    Last edited by a moderator: Oct 9, 2014
  2. jcsd
  3. Oct 8, 2014 #2


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    Homework Helper

    It's kind of puzzling why you chose to hide the problem statement behind the 'Spoiler' tag. Usually, the 'Spoiler' is to hide something which might reveal the solution.
  4. Oct 8, 2014 #3
    Because the problem was really long, and I thought it would be best to put it behind the spoiler.
  5. Oct 9, 2014 #4
    How many grams of X were there in the cylinder after decomposition?
    How much was the volume increase?
    How much volume do you estimate the liquid occupied in the cylinder before decomposition?
    What is your estimate of the final volume?
    What is your estimate of the volume of gas in the cylinder after decomposition?
    What is your estimate of the total number of moles of gas in the cylinder after decomposition?
    What is your estimate of the number of moles of X in the cylinder after decomposition?
    What is your estimate of the atomic weight of X?
    What element has this atomic weight?

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