solubility question

[Genie133]

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i had a question on a practice FL about precipitates. it asked:

if the solution containing the precipitate is stored overnight before filtration, which of the following woudl be true?

the answer was that the solution had to be protected from light to prevent photodecomposition.

one of the FALSE answers was: evaporation of the solvent will result in a decrease in the mass of precipitate recovered, and the reasoning was because it would actually result in an INCREASE in mass recovered because the smaller volume of solvent is more quickly saturated. that's the part i don't completely understand, how would the smaller volume of solvent becoming more quickly saturated result in an increase in mass of the precipitate? i'm trying to picture how that would work..

is it because less of the solvent could be carried in solution, since it's a smaller amount, so whatever is left of it would have to precipitate?... i still can't quite get a picture of it in my head.

any chem whizzes have any insight?

 

[Chowdder]

I'm not sure if I get what you're saying but smaller quantity of solvent would result in less solute being dissolved. Picture a pot of very salty water. If you boil the salty water, the water (solvent) would decrease while the salt (solute) would remain constant. As the ratio of solvent to solute becomes smaller, the solute would began precipitating out. The less solvent you have, the more solute will precipitate out.

I'm not sure if that answers the question.....😕

 

[thechairman]

you will have to provide the passage and question, or at least give the FL test #. i do recall doing this question awhile back

 

[Genie133]

it's from kaplan FL 4, quesion 34, it's talking about Silver Chloride precipitate, and it's saying that silver chloride will have a dynamic equilibrium.... the point of my question is, when you have something that is precipitating, and you take out the solvent, how would that increase the mass of the precipitate?

 

[Genie133]

I'm not sure if I get what you're saying but smaller quantity of solvent would result in less solute being dissolved. Picture a pot of very salty water. If you boil the salty water, the water (solvent) would decrease while the salt (solute) would remain constant. As the ratio of solvent to solute becomes smaller, the solute would began precipitating out. The less solvent you have, the more solute will precipitate out.

I'm not sure if that answers the question.....😕

ok i think that makes sense

 

[Omyss]

wouldn't the same amount of solute precipitate out in teh end...so the only difference is the time it would take?

 

[changarang]

no, you wouldn't necessarily get the same amount of precipitate in the end. the amount of precipitate you get depends on how much of the solute is able to dissolve in the first place. i agree with chowdder's explanation...if the solvent is evaporating, then you have less solvent, and the less solvent you have, the less solute can dissolve, which is another way of saying that more solute will precipitate out.

however, the total mass of the solute doesn't change because of the law of conservation of mass. but the question was addressing the mass of recovered precipitate, not the total mass of the solute.