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Imagine you were doing a calorimetry experiment to measure the molar enthalpy change for dissolving ammonium chloride (NH4Cl(s)) in water, H2O(l) to create an aqueous NH4Cl (aq) solution. What would you need to measure in order to calculate the molar enthalpy in kJ/mol? 

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Answer:

Volume of water

Initial temperature of the solution

Final temperature of the solution

Mass of NH₄Cl

Explanation:

There are two heat transfers involved in this experiment.

Heat of solution of NH₄Cl + heat lost by water = 0

                   q₁                      +                 q₂                = 0

                  nΔH                   +             mCΔT             = 0

                  nΔH                                                         = -mCΔT

                  ΔH                                                           = -mCΔT/n

So, you must measure

  • the volume of water (you will probably use its density to calculate its mass)
  • the initial and final temperatures of the solution (to calculate ΔT)
  • the mass of NH₄Cl (to calculate the number of moles)

You will probably assume that the specific heat capacity of the solution is the same as that of water, so you won't be measuring it.

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