AAMC #3 Physical Sciences #4

[premedinDC]

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I don't understand this question at all. I am not sure how one can determine that PbSO3 is less soluble than PbI2 (and so on) from the information in the passage. Can someone please help me out with this one.

 

[chemistryguy]

In the question, you start with lead nitrate, then add sulfate ion and get a precipitate, so you know lead sulfate is less soluble then lead nitrate (it must be to drive the reaction forward, hence you see a precipitate !!)

In the next reaction, you add iodine to the lead sulfate, and some of it dissolves!, its not soluble, so the only way it dissolves is if a metathesis rxn occurs and you get some lead iodide, which is also insoluble. From that reaction, you known lead iodide must be less soluble then lead sulfate to drive this reaction.

Lastly, the lead iodide yields a precipitate of lead carbonate when carbonate ions are present, so you know lead carbonate is less soluble than lead iodide.

Putting them in order, sulfate is most soluble, then iodide, then carbonate.

So if you add lead ions to a solution of these, the least soluble salt will precipitate first, which is lead carbonate!!

sorry for long message...

 

[SaintJude]

I'm not quite sure I understand the logic:

you start with lead nitrate, then add sulfate ion and get a precipitate, so you know lead sulfate is less soluble then lead nitrate

Why does the formation of a certain precipitate, suggests that A is "less soluble" than the starting reactant?

 

[kasho11]

It's a pretty straight forward set of displacement reactions occurring. I got this question wrong too though. Basically each time you add a chemical to the precipitate a displacement occurs because what you're adding is LESS soluble than what you had. So SO4 is initially displaced by I-, and so the precipitate is now PbI2, meaning I- is less soluble than SO4 (because it precipitated out while SO4 dissolved in solution). This happens again with CO3, it displaces I- and precipitates out. This means that CO3 is less soluble than I-, which is less soluble than SO4. So they would precipitate out, with the least soluble thing coming first, CO3, I-, then SO4.

 

[SaintJude]

It's a pretty straight forward set of displacement reactions occurring. I got this question wrong too though. Basically each time you add a chemical to the precipitate a displacement occurs because what you're adding is LESS soluble than what you had. So SO4 is initially displaced by I-, and so the precipitate is now PbI2, meaning I- is less soluble than SO4 (because it precipitated out while SO4 dissolved in solution). This happens again with CO3, it displaces I- and precipitates out. This means that CO3 is less soluble than I-, which is less soluble than SO4. So they would precipitate out, with the least soluble thing coming first, CO3, I-, then SO4.

That is the assumption in the logic that I needed to read---👍!