The purpose of this model is to demonstrate the common ion effect on the solubility of salts. In short the common ion effect states that the addition of a salt to a solution containing some common ion will decrease the degree of dissociation of a weak electrolyte in solution. This effect is especially important when creating the Barium contrast solution ingested by patients for some medical imaging procedures. The barium sulfate in this material is not harmful to the body, however some will dissociate into relatively harmless sulfate ions and toxic barium ions. In order to make the solution safe for consumption sodium sulfate is added in order to bind the dissociated barium and make the solution non-toxic.
This model will be used in conjunction with an exploratory lesson discussing the common ion effect along with the application of calculus concepts. Students will begin by researching a barium swallow and the negative effects of barium. Student will then do some pre-model calculations on the associated worksheet using the solubility product rules and their knowledge of chemistry to calculate the amount of Barium Sulfate to add to 500 ml of water in order to create a 2% wt/wt Barium Sulfate solution and the concentration of free Barium in the solution. Students will then open the project “ContrastSimulation” and the worksheet labeled “Contrast”. After pressing play students will see the Barium Sulfate molecules diffuse through the solution, with some molecules dissociating into barium and sulfate ions. Students should make note of the number of Barium ions after 30 – 45 seconds and then end the simulation. Students will then open the worksheet “ContrastAndSodiumHydroxide” and allow it to run as with the “Contrast” sheet again noting the Barium level after 30-45 seconds. The remaining worksheets should be explored in this manner starting with “ContrastAndBariumHydroxide”, then “ContrastAndPotasiumBromide”, and finally “ContrastAndSodiumSulfate” always noting the Barium level when the simulation is stopped. From exploration of the model students should realize that the salt they should add to their contrast material is Sodium Sulfate. Armed with this information and the rules for calculating concentrations based on solubility products students will then calculate the minimum amount of Sodium Sulfate to add to their contrast solution.
The primary file is a lesson plan, accompanied by supplemental files. In the supplemental zipped files, you will find:
- Student worksheets
- Lesson plan
- Powerpoint presentations
Lash, Stephanie; Burleigh, Nathanial; and Moulin, Katrijn, "Safe Imaging of Internal Organs" (2014). Lesson Plans. 345.
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