TBR Chapter 3 Solubility question :'(

[ilovemedi]

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Number #64. Completely confused as to WHAT they're asking and even more confused at the explanatio. For example, where did they assume that "the average salt has 100mg" molecular mass (answers says that)?!?!

 

[ipmed]

The answer says the concentration is 1 molar, and just look at the molar solubility because of the complication with zinc and silver forming dications. The greatest difference would be easiest to detect, assuming your not dealing with almost nothing dissolving like 10-10or<

 

[ilovemedi]

The answer says the concentration is 1 molar, and just look at the molar solubility because of the complication with zinc and silver forming dications. The greatest difference would be easiest to detect, assuming your not dealing with almost nothing dissolving like 10-10or<

Doesn't A have the greatest difference in molar solubility? 3.4*10^-17 and 1.6*10^-11.. Why is it B then?

 

[BerkReviewTeach]

Let's say for sake of argument that one tube had 0.01 M Ag+ and the other had 0.01 M Zn2+. If you were to add KCl to the solution such that it also was roughly 0.01 M or so, then you would be under the molar solubility of ZnCl2, so it would remain completely in solution and you would see nothing. But, in the Ag+ tube, you have a concentration much higher than the maximum allowed (the molar solubility is 1.2 x 10^-5 M), so it will precipitate out of solution, and you'll see the powder at the bottom of the test tube. So, adding Cl- to the two respective test tubes will result in two distinguishably different visual results, and thereby allow you to safely identify which one contained Ag+ and which on contained Zn2+.

 

[ilovemedi]

I'm still confused and feel like an idiot. I saw another explanation here that makes more sense http://forums.studentdoctor.net/threads/tbr-gchem-equilibrium-question.888987/

but I cannot for the life of me understand why they stated in the thread that:
Ag will be visible if it is < 0.01 M
and Zn will be visible if it is < 0.015 M

How did 0.01 and 0.015 came to be?

 

[lexw]

MrNeuro's answer

passage states that
INSOLUBLE TO THE EYE IS < 1 mg/mL = < 1 g/L

question is asking about Zn and Ag (Zn = 65 g/mol, Ag = ~100 g/mol)

Taking the fact that we know that a precipitate will only be visible if the solid is found at < 1g/L we can find the minimum concentration for each metal that is required to form so it becomes visible to the eye.

we know that the molar mass of Zn = 65 g/mol and Ag ~100 g/mol (107 to be precise but who needs to be precise on the mcat) we can look at the units to figure out how to get the molarity (mol/L)

for [Zn] = 1 g/L / 65 g/mol = 0.015 mol/L => 0.015 M

[Ag] = 1 g/L / 100 g/mol = 0.01 M