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Determination of the Equilibrium Constant, K sp , for a Chemical Reaction

Determination of the Equilibrium Constant, K sp , for a Chemical Reaction. By: Bronson Weston. Background Information. K sp is a particular type of equilibrium constant called the solubility product constant.

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Determination of the Equilibrium Constant, K sp , for a Chemical Reaction

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  1. Determination of the Equilibrium Constant, Ksp, for a Chemical Reaction By: Bronson Weston

  2. Background Information • Ksp is a particular type of equilibrium constant called the solubility product constant. • Equilibrium is achieved when an ionic solid dissolves to form a saturated solution. The equilibrium exists between the aqueous ions and the precipitate, an undissolved solid.

  3. A saturated solution contains the maximum concentration of ions of the substance that can dissolve at the solution's temperature. • As the concentration of solute, dissolved ions, increases, so does the rate of reprecipitation. When the rate of reprecipitation equals the rate of dissolution, and there is no more net dissolution of solid, equilibrium is reached.

  4. Ksp Equation • If given the following reaction: • AnBm(s)  n Am+ (aq) + m Bn-(aq) • The Ksp of the reaction is: • Ksp = [Am+]n[Bn-]m • Ksp = molarity of solution  solution is saturated, no precipitate • Ksp< Molarity of Solution  solution is saturated, precipitate is formed • Ksp > Molarity of Solution • solution is unsaturated, no precipitate is formed

  5. Materials • Calcium nitrate, Ca(NO3)2, 0.0900 M • Sodium Hydroxide, NaOH, 0.100 M • 96- Well Microplate • Beral pipets http://www.odec.ca/projects/2007/meng7l2/G4PicsVideoClips_006.jpg labtutorials.org www.flickr.com

  6. Procedures- Row 1 • Arrange the Microplate so that you have 12 wells across from left to right • Put 5 drops of water in wells #2 through #12 in the first row • Put 5 drops of .0900 M Ca(NO3)2 in well #1 in the first row • Add 5 drops of Ca(NO3)2 to well #2 www.gutenberg.org

  7. Using an empty Beral pipet, mix the solution in Well #2 thoroughly by drawing the solution into the pipet and then squirting it back several times • Calculate the molarity of the solution in well #2. 5 drops of 0.0900 M = n moles 5 drops 5 drops Ca(NO3)2 + 5 drops water = 10 drops n moles = 0.0900 M = 0.0450 M 10 drops 2

  8. Using the empty pipet, draw the solution from well #2 and put 5 drops into well #3 • Put the remaining solution back into well #2 • Mix the solution in well #3 with the empty Beral pipet as before. • Continue this serial dilution procedure, adding 5 drops of the previous solution to the 5 drops of water in each well down the row until you fill the last one, well #12. • After Mixing the solution in well #12, discard 5 drops http://www.wwmponline.com/cart/images/T/XPierce_close.jpg

  9. Calculations • Determine the concentration of Ca(NO3)2 solution in each well, using the method used in step 6

  10. More Procedures • Place 5 drops of 0.100 M NaOH in each well, #1- #12 • Use an empty pipet to mix the solution in each well • Calculate the concentration of each reactant, Ca+2 and OH-, and record the data on a table.

  11. Allow three or four minutes for precipitates to form. • Observe the pattern of precipitation and record, on the table, which solutions form a precipitate. I drew this one myself

  12. Assume that the first solution, the most concentrated, that does not form a precipitate represents the saturated solution. • Calculate the Ksp of Ca(OH)2, using the concentration of Ca+2 and OH- ions in the saturated solution

  13. This lab was intended to demonstrate how you can figure out a salt’s solubility constant. When a solution is saturated, it is in equilibrium. Therefore, when a solution is saturated, we can use the current concentrations of the ions to determine the Ksp. This is why we used the concentrations in the 3rd well to determine the solubility constant. We were able to assume that the 3rd well was a saturated solution, because it was the most concentrated solution that did not form a precipitate. Explanation

  14. References • http://faculty.kutztown.edu/vitz/limsport/LabManual/KSPWeb/KSP.htm • http://www.jesuitnola.org/upload/clark/aplabs.htm#Determination%20of%20the%20Solubility%20Product%20of%20an%20Ionic%20Compound • http://mooni.fccj.org/~ethall/2046/ch19/solubility.htm • Vonderbrink, Sally Ann. Laboratory Experiments for Advanced Placement Chemistry Student Edition. Flinn Scientific, Inc. Batavia, IL. 1995.

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