Determination of the Solubility Product of an Ionic Compound

AP Chemistry Equilibrium

The solubility product constant, Ksp, is a particular type of equilibrium constant. The equilibrium is formed when an ionic solid dissolves in water to form a saturated solution. The equilibrium exists between the aqueous ions and the undissolved solid. A saturated solution contains the maximum concentration of ions of the substance that can dissolve at the solution’s temperature. Knowledge of the Ksp of a salt is useful, since it allows us to determine the concentration of ions of the compound in a saturated solution. This allows us to control solution so that precipitation of a compound will not occur, or to find the concentration needed to cause a precipitate to form. The solubility product which will be determined by this experiment is that of the strong base, calcium hydroxide, Ca(OH)2.

Caution: Sodium hydroxide is a strong base and is hazardous to skin and eyes. If you get any on yourself, wash it off with large amounts of water. Neutralize spills on the counter with vinegar.

Procedure

Note: Be sure to hold all Beral droppers vertically. Make sure no air bubbles are in them. Discard the first drop as it may contain an air bubble.

Experiment 1 Step One: Serial Dilution of Calcium ion

1. Take 2 well plates and set them next to each other so that you have 12 wells left to right. Place 5 drops of 0.10 M calcium nitrate in well #1 and 5 drops of deionized water in wells #2-12.

2. Place 5 drops of calcium nitrate in well #2. Use an empty dropper to mix the solution. Its concentration is 0.050 with respect to calcium ion.

3. Use the mixing dropper to remove 5 drops of solution from well #2 and place it in well #3. Return any extra solution to well #2. Mix the solution in well # 3 as before. Continue this serial dilution procedure until you fill the last well, #12. Discard 5 drops of the solution in well #12. Determine the concentration of solution in each well as described in the calculations section.

Step Two: Combine with Sodium Hydroxide

Place 5 drops of 0.10 M sodium hydroxide in each of the wells. Use an empty dropper to mix each of these solutions or mix thoroughly with a toothpick. Note that when the sodium hydroxide is added the initial concentration of both calcium and hydroxide ions is halved because each solution dilutes the other solution.

Step Three: Observe the precipitates and calculate Ksp

1. Allow 3-4 minutes for precipitates to form. Observe the pattern of precipitation. 2. Assume that the first well with no precipitate represents a saturated solution; note the number of this well.

Perry and Emmer June 2010 AP Chemistry Equilibrium

3. Calculate the concentration of ions in the well and using these concentrations determine the solubility product of calcium hydroxide (see calculations).

Experment 2 Verify using a dilution of sodium hydroxide

1. Repeat the procedure but make a serial dilution of 0.10 M sodium hydroxide instead of calcium nitrate. (Follow the directions in Experiment 1 step one) 2. Add 5 drops of 0.20 M calcium nitrate to each well. (Follow the directions in Experiment 1 step two)

3. Observe results and calculate Ksp. (Follow the directions for experiment 1step three)

Flush all solutions down the drain. Clean well plates and lab area. Wash hands.

Calculations 1. Prepare a data table to record the value of the concentration of calcium ion in the wells after Step 1, the concentration of calcium ions in the wells after Step 2, and the concentration of hydroxide ion in each of the 12 wells after Step 2. Verify that the concentration in well # 12 is 4.9 x 10-5 M. (You must show one sample calculation). Record your values in the data table. 2. Determine the concentration of calcium ion and hydroxide ion in each well in part B. Show one sample calculation. Record your values in the data table. 3. Write the equation for calcium hydroxide dissolving in water. 4. Write the solubility expression for calcium hydroxide. 5. Use the calcium and hydroxide ion concentrations in the first well with no precipitate (see step 3) to calculate the solubility product for calcium hydroxide. Identify the well number! 7. Calculate the calcium and hydroxide ion concentrations in the first well with no precipitate in experiment 2. Identify the well number!

8. Calculate Ksp of calcium hydroxide using the data from experiment 2.

9. Calculate the molar solubility of calcium hydroxide, based on your Ksp value (#5 above)

Analysis

1. Do you expect to obtain the same results in experiments 1 and 2? Explain. 2. Look up the accepted value for the solubility product of calcium hydroxide and compare it to your values. What was your percent error? 3. What is a serial dilution? What are the advantages and disadvantages of this technique?

4. Do you think this method will give values of Ksp that are too high or too low? Explain. 5. Would your results be better if you averaged the concentrations of the last well where precipitation occurred with the first well where there was no precipitate? Is there any justification for doing this? 6. What would make this method more accurate?

Conclusion

Having completed this lab, you should have a better understanding of what Ksp is and how it is calculated. You should know why this is a valuable physical constant to know. You have also been introduced to a new technique, serial dilution.

Perry and Emmer June 2010