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Activity in Chemical Equilibrium (Mullaugh)

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    Learning Outcomes:

    1. Calculate the ionic strength of a solution.
    2. Describe the relationship between ionic strength and activity.
    3. Quantify the impact of ionic strength on the equilibrium concentration of an analyte.

    Background: Barium enema is a diagnostic medical procedure in which the inside of the large intestine is coated with an aqueous slurry of insoluble BaSO4 to facilitate X-ray imaging. However, barium is a heavy metal that’s absorbed in the gastrointestinal tract; levels as low as ~7 x 10-5 M have been shown to result in a risk of heart attack and stroke. Should you worry about having this procedure done?

    Answer the following questions regarding BaSO4 dissolved in a sodium chloride solution. Assume the NaCl is 10x more concentrated than dissolved BaSO4.

    1. As a group, illustrate and describe how any ions might arrange themselves in solution. What ions will be in close proximity to Ba2+? What ions will be in close proximity to SO42-? Be sure to consider the relative number and size of each ion.





    1. Consider the edge of the “ionic atmosphere” drawn around Ba2+ and SO42- in question 1. How does the apparent charge of Ba2+ and SO42- at this point compare to the actual charge of each ion?



    1. Based on your answer to question 2, predict how the ionic atmospheres around Ba2+ and SO42- will impact their attractive force toward one another.



    1. As a group, decide how to complete the following statement. Dissolving BaSO4 in a sodium chloride solution will cause its solubility to __________ relative to water.
      1. increase                  b. decrease                  c. stay the same


    1. In reality, the barium slurry isn’t prepared in deionized water or even a simple sodium chloride solution. It’s made in a physiological saline solution that contains:
      • 0.137 M NaCl
      • 0.0027 M KCl
      • 0.0809 M Na2HPO(dissociates into Na+and HPO42- ions)
      • 0.0176 M KH2PO(dissociates into K+and H2PO4- ions).

      Calculate the ionic strength of the physiological saline solution.




    1. What are the activity coefficients for Ba2+and SO42- in this saline solution? (αBa2+ = 500 pm; αSO42- = 400 pm) Note you will have to use the extended Debye-Hückel equation.


    1. Calculate the solubility of BaSO4 in
      1. pure water


      2. a 0.050 M NaCl solution (use the table to get activity coefficients)


      3. the physiological saline solution described above.


    1. Based on the Ba2+ concentration at which health concerns may arise (~7 x 10-5 M), decide as a group whether you should be concerned about getting a barium enema.



    1. Look over the table of activity coefficients. Recognizing that ignoring activity effects is the same as treating all activity coefficients as unity (1), when is activity most important? Circle one for each.
      1. When ionic strength is (high/low).
      2. When the sizes of ions involved in equilibrium are (large/small).
      3. When charges of ions involved in equilibrium are (large/small).

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