(adsbygoogle = window.adsbygoogle || []).push({}); 1. The problem statement, all variables and given/known data

Hello. I have a lab procedure that I am supposed to write, and since my teacher is on a two-week sick leave and the substitute does not know anything, I seriously need some assistance.

Please note that for the following problem, a titration is NOT allowed. It must be done using Ksp and Ktrials.

Here is the problem: There is an unknown solution that is either Lead Nitrate or Potassium Iodide.

a) I must devise a procedure to determine which solution it is (I know how to do this part)

b) I must devise a procedure to determine the concentration of the solution once I have determined which solution it is. There is a set list of possible concentrations that we are given.

IF it's Potassium Iodide

Possiblity 1: 0.05M - 0.15M

Possibility 2: 0.20M

IF it's Lead Nitrate

Possiblitty 1: 0.20-0.30M

Possibility 2: 0.05-0.015M

Possibility 3: >0.30M

We also given two KNOWN solutions (to use in the experiment for finding the unknown concentration sample) of both Lead Nitrate and Potassium Iodide, each having 0.01M concentration.

2. Relevant equations

Ksp PbI2 = 8.5 x 10^-9

Ktrial = [Pb]^2

3. The attempt at a solution

So I've been doing alot of brainstorming and I came to some conclusions that are based of qualitative observations.

I know how to figure out part A, which is figuring out WHAT the solution is. I must simply put some of the KNOWN lead nitrate, for example, into the solution, and if I get a precpitate, I know that the solution is Potassium Iodide. And vice versa if I decide to add Potassium Iodide

Part B is the problem. Now I realize that I can pre-calculate the Ktrial for each of the POSSIBLE concentrations of the unknown. For example, if we assume that our solution is found to be potassium Iodide, I can find out the Ktrial for both the 0.05-0.15M possibility and the 0.20M possibility. I have actually done this calculation, and I find that for the 0.05-0.15 possibility, the Q is more than the Ksp, but it is very close to it, meaning that I would see a very light precipitate, if any. With the 0.20M possibility, I have a Q substantially larger than Ksp and so I would get a dark precipitate.

Using this QUALITATIVE property, I can thus discern a distinction and be able to state the concentration (or range of concentration) that the unknown falls within.

My problem is that I do not think this to be a reliable method. I was wondering if anyone else has an idea as to how I can figure out part B of my procedure? Thanks alot

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# Homework Help: Determination of an unknown concentration

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