Molar solubility and precipitates

[ashtonjam]

This attached problem is from TBR Gen Chem, Equilibrium passage 7, #46.

I understand that adding Cl- to Ag+ solution will precipitate out AgCl, so the answer is B. According to the table, the magnitudes of molar solubility for Ag, Sr, and Zn with Cl- are 10^-5, 10^-2, and 10^-2 respectively. Adding Cl- will precipitate out Ag but not Sr and Zn, which is probably explained by the difference between 10^-5 and 10^-2 values. Is there a molar solubility cutoff point at which it is safe to say that something will precipitate or not?

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[enervate]

Yes. A compound with a molar solubility of -∞ will always precipitate and a compound with a molar solubility of +∞ will never precipitate.

 

[jtran7]

This attached problem is from TBR Gen Chem, Equilibrium passage 7, #46.

I understand that adding Cl- to Ag+ solution will precipitate out AgCl, so the answer is B. According to the table, the magnitudes of molar solubility for Ag, Sr, and Zn with Cl- are 10^-5, 10^-2, and 10^-2 respectively. Adding Cl- will precipitate out Ag but not Sr and Zn, which is probably explained by the difference between 10^-5 and 10^-2 values. Is there a molar solubility cutoff point at which it is safe to say that something will precipitate or not?

I don't have the TBR Study material and the question seems to be a bit vague but this is my way of thinking.

Since each compound has a Ksp value it means that by adding a common ion they each have the ability to precipitate out (depending on how much Cl- is added). However since the question doesn't give you additional information in terms of the concentration of the Cl- or the compound in solution, you can not determine which compound will precipitate ie using Qsp v.s Ksp. (I am assuming the question is referring the a solution that has all four compound mixed together)

Since in the question stem they tell you a precipitate form you know with 100% certainty that AgCl precipitated out because it has the smallest molar solubility of the four compounds. You just can't be sure if any of the other compounds precipitated out since they didn't tell you how much NaCl was added.

For answer C (both Sr 2+ and Pb2+ are in solution) using the same logical you can't be sure if its still in solution or not, maybe it precipitated out since they didn't tell you how much NaCl was added.

The same applies to A and D.

So when they ask you what conclusion CAN be drawn the only one is Ag+ is present.
Again this is assuming that the solution they are referring to has all 4 compounds dissolved in it.