# Ksp - Prelab help

1. Aug 2, 2006

### rachelle

Hi, I'm trying to do my prelab and I have no idea where to start... if someone can point me to the right direction, I will absolutely be grateful...

Given the following equilibrium:

PbI2 <----> Pb + 2I

You have a solution of the following:

4.77 ml of .00274M Lead Nitrate solution
4.35 ml of .00215M Iodide solution
5.00 ml of water

You measured the solution and get the following:
%T blank = 100.3 %
%T sample = 42.9 %
Absorbance of a 1.000 mM Iodide soln = .775

Calculate the following:
micromoles of Pb(II) originally put in solution
micromoles of I- originally put in solution
mM of I- at equilibrium
micromoles of I- in solution at equilibrium
micromoles of I- precipitated
micromoles of Pb(II) precipitated
micromoles of Pb(II) in solution at equilibrium
mM of Pb(II) in soluion at equilibrium
Ksp

The only ones I solved that were correct were the following:
total volume = 14.12 ml
absorbance of sample = .3688

Hope someone can help me...

2. Aug 2, 2006

### sdekivit

first: your reaction equation is not completely correct. Dissolving $$PbI_{2}$$:

$$PbI_{2} \longrightarrow Pb^{2+} + 2 I^{-}$$

for the original micromols $$Pb^{2+}$$ you have to use the formula $$c = \frac {n} {V}$$ and you use the given: 4.77 ml of .00274M Lead Nitrate solution

The same counts for micromols $$I^{-}$$

The third one you need to use the law of Lambert-Beer: $$A =\epsilon\cdot c\cdot l$$. From the given : Absorbance of a 1.000 mM Iodide soln = .775, you can calculate $$\epsilon$$ And then with this found $$\epsilon$$ you can calculate $$[I^{-}]_{eq}$$. Only you need to know l: the length of the cuvet where the light goes through. Since it's a prelab, you'll probably have this parameter.

Then you have everything to go on calculating $$K_{sp}$$

Last edited: Aug 2, 2006
3. Aug 2, 2006

### rachelle

Thank you sooo much! :rofl:

Okay, I've solved everything correctly except for Ksp :surprised

I should use Ksp = [Pb]^2 right?

But which concentrations should I use? The ones at equilibrium? Do I need to do "ICE"? I'm not familiar with it, but I searched and they mentioned "ICE", not sure if it's applicable here though...

Thanks again!

Last edited: Aug 2, 2006
4. Aug 2, 2006

### sdekivit

yes, but watch your notations !

$$K_{sp} = [Pb^{2+}(aq)][I^{-}(aq)]^{2}$$

you already know the equilibrium concentrations and you need them here. As for your question about ICE: you already had to use it to get to the equilibriumconcentrations.

5. Aug 2, 2006

### rachelle

Thank you, thank you, thank you~~sdekivit!! I finally got it

Also yes, I should learn how to use the notations correctly... I'll read up on how to use the notations here on the forum
Thank you again!!!!

6. Apr 16, 2007

### ecuafrap

Hi, i couldn't help but wonder how you calculated the equilibrium concentration of I- from the molar extinction coefficient, E?? I have a very similar problem to Rachelle's post...