The entropy change for a process can be determined experimentally in a variety of ways. One of these is to determine enthalpy and free energy values at a constant temperature and to solve for DS. For the reaction below, the Ksp value was determined by evaporating 750.0mL of a saturated solution to dryness and measuring the mass of the remaining solid. The DH value was determined using temperature change data when samples of the solid were dissolved in 50.0mL of pure water (recall that the specific heat of water is 4.184 J/g*oC). Assuming that all data were collected at 298K, determine the DS for the process:

Question

The entropy change for a process can be determined experimentally in a variety of ways. One of these is to determine enthalpy and free energy values at a constant temperature and to solve for DS. For the reaction below, the Ksp value was determined by evaporating 750.0mL of a saturated solution to dryness and measuring the mass of the remaining solid. The DH value was determined using temperature change data when samples of the solid were dissolved in 50.0mL of pure water (recall that the specific heat of water is 4.184 J/g*oC). Assuming that all data were collected at 298K, determine the DS for the process:

NH4NO3 (s) ---> NH4+ (aq) + NO3-1 (aq)

Masses after Evaporation:
Trial: Mass:
1 1.203g
2 1.237g
3 1.252g

Temperature Changes:
Trial Mass dissolved Tinitial Tfinal
1 4.063g 23.9oC 18.2oC
2 3.997g 24.2oC 18.6oC
3 4.112g 23.8oC 18.0oC

Answer 1

John, I don''t understand what you've done. Is this a problem or is it a lab experiment? First, I never heard of NH4NO3 having a Ksp. However, if that is a valid number you can use dG = -RTlnK to calculate dG. Then the evaporation data is supposed to give you dH and that along with

dG = dH - TdS will give you dS @ 298 K.
Now for the dH value. That's the part I don't understand. How was the saturated solution made? And if you placed almost 4 grams NH4NO3 to make the saturated solution why did you get only 1.2 or so grams back after evaporation. Anything you can provide to help clear this up is appreciated.