Ksp and Q precipitate, Why does volume of water matter

[AMan24]

Homework Statement

Homework Equations

The Attempt at a Solution

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So here's my question again, why do you need to include 40L of water in the total volume? It's not like AgCl is going to dissolve in water right? AgCl has low solubility in water. I mean if it was like NaCl then you should include the water because it dissolved, but if it was something that isn't soluble in water at all, then why would you include the water?

Lets say you did a reaction like this and it formed something that isn't solube in water at all, if you include the water in the math, the math will say no precipitate formed, but in reality there will be a precipitate formed.

So for this reaction i'd say a precipitate will form but a small part of it would be dissolved

 

[Borek]

It is Q that defines what will precipitate and what will not. To calculate Q you need concentrations. How are you going to calculate concentrations without taking the final volume into account?

 

[AMan24]

Does it have anything to do with water being added in the beginning? So a precipitate never forms in the first place because the concentrations are too low, if it is then i guess i kind of understand it

 

[Borek]

Why would the situation be different if the water was added later? Why do you think AgCl would not dissolve? It may have low solubility, but it still follows Ksp. Plus, the final equilibrium doesn't depend on the path followed to get to the final state. It sometimes does, when reactions are irreversible, but that's not the case here.

 

[Dr Uma Sharma]

what is Ksp of AgCl ?
it is the ionic product of [Ag+ ] and [Cl-] .These are the concentration of ions in solution and solution means solute (AgCl) and solvent( water here).It is not that AgCl is completely insoluble, its very low Ksp indicates that it is very less soluble.It means volume of water added do matter in precipitation of AgCl and cases will be there where Q<Ksp just because of excess water...like in your problem.