Determination of the Ksp of Calcium Hydroxide Abstract In this experiment, a Ca(OH)2 solution was made and then experimented on to find what it’s Ksp is. Three different methods were used called the gravimetric determination, pH, and Titration. Through the methods used, the Ksp of each were as follows: 3.15x 10­7, 5.832x 10­6, and 6.52x 10­5 respectively. The pH method was the most accurate and seems most reliable; percent errors for each method were 93%, 16%, and 30% respectively. Introduction Calcium Hydroxide is a strong base that always dissociates into Ca2+ and OH­. In this experiment, a solution containing Calcium Hydroxide and water was made to conduct three different methods of determining what Calcium Hydroxide’s Ksp is. Calcium Hydroxide’s dissociation looks like below: Ca(OH)2(s) ↔ Ca22+ (aq) + 2OH­(aq) Doing the three different methods will allow the class to find the mass of dried Calcium Hydroxide, The pH of the Calcium Hydroxide, and the pH of HCl, that will be titrated into the calcium solution. In doing so, work can be done to work to find either the [Ca2+] and/or the [OH­] and then those values can be plugged into finding the Ksp. The equation to find the Ksp is below. Ksp=[Ca2+][OH­] Data Method 1 Mass of Dried Ca(OH)2 (g) Liters of solution Concentration of Concentration of Ksp Ca(OH)2 Ca2+ (M) (M) .008 .01 .0108 .0054 3.15x 10­7 Method 2 pH Ksp Ca(OH)2 12.27 5.832x 10­6 Method 3 Change in mL Concentration of Liters of solution Concentration of Ksp HCl OH­ (M) (M) 8.05 .05 .01 .04025 6.52x 10­5 Results During method 1, after the mixed solution was put into the oven and dried, the mass of the dried Calcium Hydroxide was .008g. With this mass, you can determine the concentration of the Calcium Hydroxide and use an ICE table to find concentrations of Calcium and Hydroxide. Which, when multiplied, will give the Ksp which is 3.15x 10­7 . During method 2 the pH reading of Calcium Hydroxide was found. With this known value, the OH­ concentration can be found thus finding the Ca2+ concentration can be found as well. When using the Ksp equation, we can find the Ksp which was 5.832x 10­6. During method 3 a titration between HCl and Ca(OH)2 took place. The concentration of the HCl added can be found. With that information you can find the OH­ concentration and then find the Ksp using the equation. The ksp was 6.52x 10­5. The average Ksp for Ca(OH)2 is 2.37x 10­5. Conclusion After doing these experiments, it was found that method 2, pH of Ca(OH)2, was the most accurate of the three method used to find the Ksp. The percent error of method 2 was only 16% and the others were 93% and 30%. These values were a lot more off than method 2. The reason that method 2 was more accurate was because there were no changes in volume or mass. This method was just measuring the pH of the solution so there should have been minimal to no error. If the buret was inadvertently left wet with water from cleaning then the Ksp would go down. This is because the acidity of HCl would slightly change and that would cause for a more diluted solution when the titration takes place. The less acidic the acid is the more OH­ will form which means it would have a lower Ksp. If phenolphthalein was used as the indicator instead of methyl red then the pH would not change in value because phenolphthalein would just turn the solution purple and the adding of an acid would just turn it colorless. So when the solution turns colorless than you can say the equivalence point was met.