Discrepancy between TBR and AAMC #3

[seraphkz]

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Okay guys I really need your help on AAMC #3 question #4

So according to AAMC:
During a separation by precipitation procedure, the last one that gets precipitated is the LEAST soluble.

However according to TBR on page 195 General Chemistry Part I:
"The least soluble salt precipitates from solution FIRST". So the last one that gets precipitates is the MOST soluble...

Which one is right? 😵....

 

[vsl5]

the one that precipitates first is the least soluble because it takes the lowest concentration of ion to exceed Ksp. But be careful if you are using multiple species to precipitate out different salts... I am assuming that you use one species in this question eg. Cl- to precipitate out salts that share a common ion like AgCl and PbCl

 

[seraphkz]

Thanks for the explanation. That's what I thought. I don't know what I'm doing wrong on AAMC #3 Question #4... Would you mind taking a look?

 

[Rabolisk]

First, AAMC's explanation is entirely consistent with BR's. You probably misunderstood the passage.

You have lead nitrate, which is soluble. You add sodium sulfate to it. Lead sulfate is not soluble, so it precipitates out. You take this lead sulfate out and add potassium iodide to it. Now, you have lead ions, potassium ions, sulfate ions, and iodide ions. Both lead iodide and lead sulfate are insoluble, but to different degrees. The fact that lead iodide precipitated out tells you that it is less soluble, or that its molar solubility is lower. Now you take this lead iodide and add sodium carbonate to it. By the same reasoning as above, lead carbonate must be less soluble than lead iodide.

Now to the question. This time you have bunch anions (and soluble cation like sodium). When you add lead cations, the first to precipitate out will be the least soluble salt, or the salt with the least molar solubility. The passage tells us that it is carbonate, followed by iodide, followed by sulfate.

 

[seraphkz]

Ah I see. Sorry for the trouble.

I think I probably misread the passage. It was a little confusing for me. I think next time I might write down a flow chart to help myself with this type of problem. I usually have 20 minutes to spare on PS anyway.

Thanks again. You've been extremely helpful!!!