NYB LAB EXERCISE 7 Solubility, Solubility Product and the Common Ion Effect Introduction The extent to which sparingly soluble salts dissolve in water is indicated either by the solubility or by the solubility product of the salt. The solubility of a salt is often expressed as a concentration, that is, as moles of salt per litre of solution. The solubility product of a salt is the product of the ion concentrations in a saturated solution, each raised to the power of its coefficient in the balanced equation. The purpose of this experiment is to determine the solubility and the solubility product of potassium bitartrate (KHC4H4O6). This will be done both in water, and in one of several solutions of potassium nitrate (KNO3) which contain a known concentration of potassium (K+) ion. The calculated solubilities and solubility products will then be compared. Potassium bitartrate is sparingly soluble in water and dissolves to give potassium ions and bitartrate ions. KHC4H4O6 (s) K+(aq) + HC4H4O6−(aq) (1) When potassium bitartrate is dissolved in water, the potassium ion concentration equals that of the bitartrate ion. In a saturated solution this concentration is equal to the solubility, x, of the potassium bitartrate which can be expressed as moles of potassium bitartrate per litre of solution. [K+] = [HC4H4O6−] = x = solubility in distilled water (2) − That is, the only way x moles of K+ and x moles of HC4H4O6 ion can appear in a litre of solution is if x moles of KHC4H4O6 dissolve. The solubility product is the product of the concentrations (in moles per litre) of the two ions in the saturated solution. Ksp = [K+] [HC4H4O6−] (3) If KHC4H4O6 is added to a solution of a potassium salt such as potassium nitrate (KNO3), then the bitartrate ion concentration in the saturated solution is not equal to the concentration of potassium ion. + − I.e. [K ] [HC4H4O6 ]. The new solubility of KHC4H4O6 is still numerically equal to the concentration of bitartrate ions in solution since all the bitartrate ions come from the dissolution of the potassium salt. However, the total potassium ion concentration is the sum of the concentration from dissolved potassium nitrate plus the concentration from the potassium bitartrate which is dissolved in the solution. Mauro Di Renzo & Vanier College Chemistry Department Page 1 of 5 Ver. 1.0.4 [K+]Total = [K+]KNO3 + [K+]KHC4H4O6 (4) The solubility product for a saturated solution of KHC4H4O6 in a solution of KNO3, can thus be written as: Ksp = [K+]Total [HC4H4O6−] (5) Since Ksp is a constant at a given temperature, it follows that the two Ksp values, obtained from equations (3) and (5) respectively, should be the same. The average value of these two is taken at the end of the experiment to be the solubility product of potassium bitartrate at that particular temperature. The bitartrate ion (HC4H4O6−) behaves as an acid and therefore its concentration can be determined by titration with a strong base: OH−(aq) + HC4H4O6−(aq) H2O() + C4H4O62−(aq) (6) Knowing the concentration of the hydroxide ion, the concentration of the bitartrate ion can be calculated. The solubility of potassium bitartrate in water, (and hence the Ksp value), is temperature dependent: thus all solubility determinations should be done at the same temperature. Pre-Lab Questions 1. Which is the common ion in this experiment? 2. What is the difference between solubility and the solubility product of a substance? 3. How do you expect the solubility product of KHC4H4O6 to vary between the distilled water solution and the KNO3 solution? 4. How do you expect the solubility of KHC4H4O6 to vary between the two solutions? Procedure Pour about 150 mL of distilled water into a 500 mL Erlenmeyer flask and add an excess (approximately 2 grams) of potassium bitartrate. Mix the contents of the flask thoroughly for 5 minutes, then let the mixture stand for 5 minutes more. Record the temperature of the solution. Filter the solution through a dry filter paper in a dry funnel, into a dry 250 mL beaker. The apparatus used here must be dry to avoid dilution of the saturated solution. Rinse and fill a clean burette with a standard sodium hydroxide solution. The precise concentration will be available in the lab. Pipette 25.00 mL of potassium bitartrate solution into a clean 125 mL Erlenmeyer flask for the titration. Add 3 drops of phenolphthalein indicator and titrate the bitartrate solution with sodium hydroxide to a permanent pale pink endpoint. Repeat the titration. If results aren’t within 0.10 mL, repeat the titration a third and final time. Mauro Di Renzo & Vanier College Chemistry Department Page 2 of 5 Ver. 1.0.4 Repeat the experiment above replacing the distilled water in the solution by the assigned potassium nitrate solution. The flask can be rinsed with the assigned potassium nitrate solution (not water!) before preparing the actual solution. Prior to titrating, note the temperature of the solution; if this is not within 1C of your first experiment, warm the flask with your hand or cool it under the tap, as appropriate. Collect titration results from other students (at least two more groups) who have used potassium nitrate solutions of different concentrations. Use the collected results to calculate the solubility and solubility product of potassium bitartrate in both water and in the potassium nitrate solutions. Plot a graph of the solubility of potassium bitartrate against the potassium nitrate concentration. Mauro Di Renzo & Vanier College Chemistry Department Page 3 of 5 Ver. 1.0.4 CHEMISTRY NYB DATA SHEET EXPERIMENT 7: SOLUBILITY PRODUCT Student Name: __________________________ Student Name: __________________________ Concentration of assigned KNO3 solution: ________ Concentration of NaOH titrant: ________ Titration Water Trial 1 Trial 2 KNO3 solution Trial 3 Trial 1 Trial 2 Trial 3 Volume of analyte Initial burette reading Final burette reading Volume of NaOH used Moles of NaOH Moles of HC4H4O6− in saturated solution [HC4H4O6−] in saturated solution Solubility of KHC4H4O6 [K+] in saturated solution Ksp of KHC4H4O6 Average Ksp Mauro Di Renzo & Vanier College Chemistry Department Page 4 of 5 Ver. 1.0.4 Results from other groups: [KNO3] Solubility of KHC4H4O6 Mauro Di Renzo & Vanier College Chemistry Department Page 5 of 5 Ksp Ver. 1.0.4