Question

When solving for the Ksp of calcium hydroxide, I had a very large percent error. I was wondering if someone could give me examples of what could cause such a high percent error when titrating Ca(OH)2 with HCl and phenolphthalein indicator. Any help would be appreciated!

Answer 1

There is a reason getting such a huge eror in your readings.

When the solution contained some solids, it is known that it is saturated. Since this solution was saturated it was meant that it was at equilibrium. This meant that the dissolving and crystallization reactions happened at an equal rate and simultaneously. For these reactions to be equal, some precipitate would have been present, indicating the saturated solution was at equilibrium.

It was necessary to remove all of the solid Ca(OH)2 before titrating because the it would have affected the rate of reaction. When the HCl was titrated with Ca(OH)2, some of the solid would go down with the liquid because of a minuscule amount of filtering.. Since the solids were really concentrated, it would have many reactions with the HCl. Therefore, the amount of Ca(OH)2 had to be only liquid, with no solid pieces going into the burette, which would have neutralized the acid too quickly. Filtering the Ca(OH)2 can’t be done perfectly with the equipment used in this lab. Without a pure solution (of Ca(OH)2) the amount of the solution would change that was needed to titrate the HCl. The value of solution went up thus giving a different value then was first assumed for Ksp. The less that was filtered lead to a higher value for the Ksp then first thought.