[benji]

Hello, i'm new to this forum and have a question about solubility (figured that is physical chemistry? not sure...)

My question is, if you have multiple substances in a water solution with varying solubility, how would the solution behave given a limited amount of water?

To make the question clearer i'll give some solubility numbers as an example:
Molecule 1 - 0.01g/mL in water at 25C
Molecule 2 - 0.10g/mL in water at 25C
Molecule 3 - 1.00g/mL in water at 25C

Theoretically, if you added 1g of each into just 1mL of water at 25C, would the most soluble molecule (number 3) take up all the water or would there be some sort of equilibrium of all three molecules in solution?

I also apologize if this is in the wrong section  now that i've read some other posts...

[jeffmoonchop]

you would probably have a ratio of each molecule dissolved in the water. Most likely a ratio of 1:10:100. You can test this by filtering the solution and weighing the solid left over. Or doing a HPLC / GC of the solution after filtering. The solid should have a composition in the inverse ratio 100:10:1.

[Arkcon]

First of all, a beginning student has to know of the common ion effect.  That is, just calling them molecule 1, 2, 3, doesn't help us give a good answer.  But sodium chloride and potassium chloride and sodium nitrate do prevent each other from dissolving.

Generally, what jeffmoonchop: said applies, for dilute solutions you can dissolve things independently of each other.

However, your instincts are good.  As you dissolve more and more solute, water molecules do end up "tied up" as it were, and not available to dissolve something else.  As an example, a saturated solution of sodium chloride does dissolve noticeably less sucrose than pure water does.

[benji]

Thanks for the quick replies!

So barring any common ion effect, the water will "prefer" a more soluble molecule over a less soluble one right?

How about caffeine C8H10N4O2 (2g/100mL) and salt NaCl (36g/100mL)? If you put 36g of salt in 100mL of water that has caffeine dissolved in it, would it precipitate the caffeine out?

[Arkcon]

Yes.  Not much in the case of caffeine, but a tiny bit of solute will come out.  This "salting out" is most noticeable for large complex molecules, such as proteins which are only soluble because the sum of their various groups interactions with water allow it to.  You can get much of a soluble protein out of solution by saturating its water solution with a very soluble salt, such as ammonium acetate.