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Finding Molar Heat of a Solution

  1. Mar 2, 2006 #1
    Hello,

    I am doing a lab currently where we are required to find the Molar Heat in kJ / mol of salt dissolved of the salt, NH4Cl

    To start the experiment, I dissolved the salt into an amount of water (values below) and recorded temperature changes.

    Mass of NH4Cl = .9 g
    Mass of H2O = 23.5 g (mL of water as well)
    Temperature initial of Water = 21°C
    Temperature final of Solution = 19°C

    Procedure: I placed the 23.5 mL of water in a styrofoam cup, took the temperature and then added the salt. I mixed the salt around and took the temperature again at it's lowest point. I know the specific heat of water is 4.186 J / g°C. I do not know the specific heat of NH4Cl.

    I am unsure where to go from here. Do I find the specific heat of NH4Cl online and then solve for its temperature initial? Am I doing the experiment wrong or is there another experiment to find the Molar Heat of the Solution (defined to me as the amount of heat given or taken from the salt)? :confused: Thanks for the help.
     
  2. jcsd
  3. Nov 9, 2016 #2

    Bystander

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    Since you've not been instructed to measure an initial temperature for sal ammoniac, you are faced with a necessity/ies to make assumptions; assumption a) could be that the final solution is dilute enough that its heat capacity is equal to that of water, and ΔHsol'n = 24.4 x 4.186 x 10-3 kJ x 2.00 /(0.9 mol/53.5).
     
  4. Nov 12, 2016 #3

    rude man

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    1. compute how many moles n of NH4Cl you have
    2. Compute the heat lost by the water
    3. answer = C ΔT/n
    where C = water heat capacity.
    This is approximate & assumes mass of solute << mass of solvent.
    This is chemistry a field in which I'm no expert so take it with the proverbial grain of NaCl.
     
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