Calculating the Ksp of Calcium hydroxide at room temp

[Zoey Brown]

Homework Statement

I conducted a titration experiment in which 0.1g of KHP was dissolved in 50mL of distilled water and placed in an Erlenmeyer flask. I filtered a Ca(OH)2 solution (that was left out open on the lab benches for us to use) and put that into the burette and titrated. Ultimately, the Ksp i calculated was 3.0 x10^-7 whereas the accepted Ksp value is 5.02 x10^-6. This gave me a percent error of 94%. I'm not as concerned about the fact that i got a huge error as much as i don't understand why. Some of the possible errors i can think of are that the temperature was cooler than 25 degrees and that the solution wasnt filtered properly. The issue is that both of those errors would have caused higher Ksp values than the accepted, not lower like mine.

Homework Equations

c=n/V
Ksp= [Ca 2+][OH -]^2

The Attempt at a Solution

I know that if my Ksp is lower, my concentrations must be lower which means i used too much calcium hydroxide. i don't know why however

 

[Borek]

How precisely was the KHP weighted? Was it correctly dried before use? What indicator was used?

 

[Zoey Brown]

the KHP was measured exactly each trial, all proper titration procedures were followed, and phenolphthalein was used. However, in any case i doubt such errors would result in such a large deviation from the theoretical value

 

[Borek]

Separately not, but they could combine producing a larger error than expected.

If the solution was left in the open it was partially neutralized by CO₂ before you started the titration.

 

[Zoey Brown]

So if the solution was partially neutralized by carbon dioxide, would that mean that it was less basic and therefore more of the calcium hydroxide solution would have to be used in order for it to be standardized by the KHP? And that would be a cause for the increase in volume used, which decreases concentration, and creates a lowe Ksp than the theoretical value?

 

[Borek]

Yes.