Question about AAMC 3R Test

[goleafsgo]

I have a question on AAMC 3R, Passage 1, Question 4:

How do you know which ion gets precipitated first? I chose B, which is the opposite, because rxn 2 shows that PbI2 formed, and then rxn 3 shows that PbCO3 formed. Doesnt that make PBI2 less soluble than PbCO3 because it precipitated out first?

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[QofQuimica]

I have a question on AAMC 3R, Passage 1, Question 4:

How do you know which ion gets precipitated first? I chose B, which is the opposite, because rxn 2 shows that PbI2 formed, and then rxn 3 shows that PbCO3 formed. Doesnt that make PBI2 less soluble than PbCO3 because it precipitated out first?

I don't have the question in front of me, so I can't comment on it exactly. But in general, you can tell which salt precipitates out first by comparing their Ksp values; the salt with the lowest Ksp will undergo the least dissociation, and therefore it will precipitate first.

 

[goleafsgo]

I don't have the question in front of me, so I can't comment on it exactly. But in general, you can tell which salt precipitates out first by comparing their Ksp values; the salt with the lowest Ksp will undergo the least dissociation, and therefore it will precipitate first.

yes, but in this specific passage, there are no Ksp's given

 

[QofQuimica]

yes, but in this specific passage, there are no Ksp's given

Ok, then they must have given some other kind of info that would allow you to make the comparison. Did they give you the molar solubilities of each ion?

 

[goleafsgo]

Ok, then they must have given some other kind of info that would allow you to make the comparison. Did they give you the molar solubilities of each ion?

they give you a series of reactions, with different precipitations

can someone who took 3R please take a look? it's killing me!

 

[QofQuimica]

they give you a series of reactions, with different precipitations

can someone who took 3R please take a look? it's killing me!

I'm going to move your thread to the main MCAT forum so hopefully one of the other students can help you.

 

[Nutmeg]

they give you a series of reactions, with different precipitations

can someone who took 3R please take a look? it's killing me!

I don't want to post anything copyprotected here, so check your PMs for a response.

 

[vmp200]

I don't want to post anything copyprotected here, so check your PMs for a response.

I was also confused on that same passage also, the explanation did not make any sense to me. So if you could please PM me your explanation.

thanks
VP

 

[yalie06]

i was very confused by this question at first too. my (not-so-sophisticated) understanding is that since PbCO3 precipitates last while the previous precipitates are converted (are not precipitates anymore), CO3 is therefore the least soluable one as the "ultimate precipitate." Hope this helps.