Ksp Pre-Lab Help: Calculate Pb(II) & I- at Equilibrium

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Discussion Overview

The discussion revolves around calculating various concentrations and the solubility product constant (Ksp) for the equilibrium involving lead iodide (PbI2). Participants are addressing a pre-lab assignment that requires them to determine initial and equilibrium concentrations of lead (Pb(II)) and iodide (I-) in a solution, as well as Ksp, based on provided measurements and chemical equations.

Discussion Character

  • Homework-related
  • Technical explanation
  • Mathematical reasoning

Main Points Raised

  • One participant expresses uncertainty about how to start the calculations for the pre-lab assignment.
  • Another participant corrects the reaction equation for the dissolution of PbI2, stating it should be PbI2 <----> Pb2+ + 2I-.
  • There are suggestions to use the formula c = n/V to calculate the original micromoles of Pb(II) and I- based on the provided concentrations and volumes.
  • Participants discuss the application of the Lambert-Beer law (A = ε·c·l) to find the equilibrium concentration of I- and note the importance of knowing the path length (l) of the cuvette.
  • One participant confirms they have solved most calculations correctly but is unsure about how to apply the ICE (Initial, Change, Equilibrium) method for Ksp calculations.
  • Another participant affirms the use of Ksp = [Pb][I-]^2 and emphasizes the need to use equilibrium concentrations.
  • A later reply questions how to calculate the equilibrium concentration of I- from the molar extinction coefficient, indicating a need for clarification on this aspect.

Areas of Agreement / Disagreement

Participants generally agree on the approach to calculating concentrations and Ksp, but there remains uncertainty about the application of the ICE method and the specifics of using the molar extinction coefficient. No consensus is reached on the best method for calculating equilibrium concentrations.

Contextual Notes

Participants mention the need for specific parameters, such as the path length of the cuvette, which may not be provided. There is also an indication that some participants may not be familiar with the ICE method, which could affect their calculations.

rachelle
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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... :redface:

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... :cry:
 
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rachelle said:
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

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

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:
sdekivit said:
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}


Thank you sooo much! :smile:

Okay, I've solved everything correctly except for Ksp

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


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.
 
sdekivit said:
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.


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

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 :smile:
Thank you again!
 
sdekivit said:
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}


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...