Does any one remember why we ignore

[Deepa100]

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liquids and solids when we write equations for dissociation constants? I think we treat them as "1" but why?

 

[andafoo]

they are not aqueous so are considered to be pure substances. Hence [??] = 1. Voila

 

[QofQuimica]

liquids and solids when we write equations for dissociation constants? I think we treat them as "1" but why?

The concentrations of pure solids and pure liquids are basically unchanged in dissociation reactions. This is because so little of a sparingly soluble salt dissociates, and because there are usually many more moles of water compared to the number of moles of solute. Since we only care about the reaction species concentrations that are changing, we lump the rest of them together into a constant. Thus, the concentration of pure solid or pure liquid is not actually equal to one, but rather is incorporated into the equilibrium expression as part of the constant (i.e., as part of K).

 

[Deepa100]

The concentrations of pure solids and pure liquids are basically unchanged in dissociation reactions. This is because so little of a sparingly soluble salt dissociates, and because there are usually many more moles of water compared to the number of moles of solute. Since we only care about the reaction species concentrations that are changing, we lump the rest of them together into a constant. Thus, the concentration of pure solid or pure liquid is not actually equal to one, but rather is incorporated into the equilibrium expression as part of the constant (i.e., as part of K).

Thanks, Q. I wish I had you as a prof for my gen chem🙂

 

[engineeredout]

Hell I don't even remember learning half this crap in gen chem the first time around.

 

[Vihsadas]

Hell I don't even remember learning half this crap in gen chem the first time around.

Me neither (hence the GPA). At least I learned it eventually though.

 

[QofQuimica]

Thanks, Q. I wish I had you as a prof for my gen chem🙂

Heh. Usually my students only said that the semester *after* they had me, not while they were in my class, and certainly not before.

with your MCAT, Deepa. 🙂

 

[andafoo]

The concentrations of pure solids and pure liquids are basically unchanged in dissociation reactions. This is because so little of a sparingly soluble salt dissociates, and because there are usually many more moles of water compared to the number of moles of solute. Since we only care about the reaction species concentrations that are changing, we lump the rest of them together into a constant. Thus, the concentration of pure solid or pure liquid is not actually equal to one, but rather is incorporated into the equilibrium expression as part of the constant (i.e., as part of K).

I was wondering if this held for rate law as well? Also, I've read in a few places about "sparingly soluble salts". Is everything we will encounter a sparingly soluble salt? Are there any examples of salts that dissociate to a large extent? I've never really seen an Ksp values that are large...

I actually had a question where they asked two molecules having the same Ksp.. which if any would have the high solubility. From the equations worked out... one species' solubility was cubed-root(Ksp/4) while the others was square-root(Ksp). That means the depending on the Ksp value either could be more soluble.

However, one of the species was said to be more soluble at all times due to the sparingly soluble arguement... being that the Ksp were always a tiny number.

Hope ya'll can understand what I'm saying 😛.