[amallyllej]

So--I feel I'm overthinking this but it's really, really getting on my nerves.

Apparently, we're to calculate the solubility product constant of PbI₂ by using a lab where we added varying amounts of Pb(NO₃)₂ and KI and watched for a precipitate. Our group's appeared after 2.0 mL of 0.010M Pb(NO₃)₂ and 1.0 mL of 0.020M KI, though we did have intervals between trials of 1 ml for Pb(NO₃)₂ and 0.5 for KI.

Our first questions were simple; net ionic equation for the precipitate formed (Pb²⁺ (aq) + 2I^- (aq) -> PbI₂ (s))and the K_sp ([Pb²⁺][I^-]²) but the rest is Greek to me despite my A on this unit's test. I feel I'm doing one thing wrong or misinterpreting the question.

For the next question, I calculated the moles of Pb²⁺ by converting mL to L, and converting 0.002 L to moles using 0.010 M. I got 2.0 X 10^-5. Then, I had to find the concentration of Pb²⁺ in the whole solution we used (which had a total of 0.01 L after adding water), so I divided moles by liters to get 0.002.

The next question was similar but with KI. I followed the same basic concepts as the above paragraph, but because there was 0.001 L converted to moles using 0.020 M, I got the same number of moles as Pb(NO₃)₂. The concentration out of 10 mL is again requested, but when I convert the moles, I divide by liters and get the same answer as above; 0.002.

I know that PbI₂ has twice the moles of I as that of Pb, so I even tried to divide Pb in half and plug everything in to the K_sp. However, I got [0.010][0.020]² which is 4 x ^-9 than the accepted K_sp value. The accepted value is 1.39 x 10 ^-8.

Where am I going crazy?

[Borek]

Don't try to fit your results to the accepted value, that's against the idea of experimental determination of the value.

Calculate Q (reaction quotient, in your case identical with Ksp) for both the mix where you have observed the precipitate and the mix where there was no precipitate, but Q was the largest smaller than the one with precipitate. Real Ksp is somewhere between these values.

PbI₂ composition doesn't matter, only things that matter are concentrations of ions used in Q.

[amallyllej]

Thank you! Though our teacher hadn't taught us that, I wish she did. I ended up just using my goofy numbers to get a 57.6% error, which is farther from the accurate value than it usually is in our labs.