Calculating the Solubility Product Constant of CaSO4 - Faculty web ...

Calculating the Solubility Product Constant of CaSO 4Prepared by Thomas M. Moffett Jr., Plattsburgh State University, 2002Background: Review solubility in your text.Insoluble salts will ionize to a very slight degree in water. In this experiment solid CaSO 4 will bein equilibrium with the ions which make it up:CaSO 4 (s) ⇔ Ca 2+ (aq) + SO 4 2- (aq)The calcium ions present in solution will react with EDTA. The endpoint of the reaction can bevisualized with the use of an indicator. When Ca 2+ ions are present they will form a red complexwith the indicator. When enough EDTA has been added, there is no longer any free Ca 2+ for theindicator to bind with and the indicator will be blue. The reaction is as follows:Ca 2+ - Indicator + EDTA --------> Indicator + EDTA- Ca 2+(red)(blue)You will first titrate a solution containing a known concentration of Ca 2+ in order to determinethe concentration of the EDTA. You can then use the concentration of EDTA to determine theconcentration of the calcium ions and then the solubility product constant (K sp ).Procedure:1.) The bench can do this step as one group: Prepare a saturated CaSO 4 solution, bydissolving approximately 1 g of CaSO 4 in approximately 100 ml of water. Put thesolution on a stir plate, and allow the solution to stir for at least 10 minutes.2.) Prepare EDTA solution by adding 0.750 g of EDTA, 100 mL of DI water, 2 crystals ofMgCl 2 , and 2 NaOH pellets to a 200 mL volumetric flask. Swirl until dissolved. Fill tothe 200 mL mark with DI water.3.) Standardize your EDTA solution by titrating a standard CaCl 2 •2H 2 O solution: Place 10mL of the CaCl 2 •2H 2 O solution into an Erlenmeyer flask. Add 3 mL of pH 10 buffer and6 drops of Eriochrome Black T indicator to the flask.4.) Fill a buret with the EDTA solution. Slowly add the EDTA to the standard CaCl 2 •2H 2 Osolution until the endpoint is reached. Repeat one time.5.) Filter the CaSO 4 solution by gravity filtration. Both groups will share the filtrate.6.) Add 5 mL of the filtrate and 20 mL of DI water (for volume) to an Erlenmeyer flask. Add3 mL of pH 10 buffer and 6 drops of Eriochrome Black T indicator to the flask. Titratethe sample with EDTA. Do a total of 4 titrations.

Post Lab Questions1.) Your calculated K sp most likely came out significantly greater than the published value. Thisis due to the fact that the sulfate ion can act a base. Write a net-ionic reaction showing sulfateacting as a base, and explain why this would lead to an increase in your calculated K sp . (hint:remember Le Chtelier’s Principle)2.) If the pH was decreased, would solubility of CaSO 4 increase or decrease?3.) A CuCl solution is prepared, where [Cu + ] = [Cl - ] = 2.0 X 10 -4 M. Is the solution atequilibrium? Explain4.) Calculate the solubility of AuI 3 when it is added to a 0.750 M Au(NO 3 ) 3 solution (include allreactions).Pre-Lab Questions1.) What does EDTA stand for?2.) Write the K sp expression for CaSO 4 .3.) Which of the following would increase the solubility of AgCN (s)?a.) Adding a strong acidb.) Adding NH 3 to form complex ions with Ag +c.) Adding NaCN (aq)d.) Both a and b4.) The molar solubility of a slightly soluble ionic compound M 2 X 3 is 2.8 X 10 -6 M. Determinethe value of K sp5.) What is the purpose of the eriochrome black T in this experiment?