Molar solubility vs. Ksp

[yellowjellybean]

Hello all,

I've read countless threads but can't seem to get an answer to my question. Why can't you use the Ksp to compare solubility? Doesn't that tell you the same thing as the molar solubility? I mean, we do use the Ka to compare the acidity. Thank you.

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[BestDoctorEver]

Hello all,

I've read countless threads but can't seem to get an answer to my question. Why can't you use the Ksp to compare solubility? Doesn't that tell you the same thing as the molar solubility? I mean, we do use the Ka to compare the acidity. Thank you.

Ksp tells you the same thing when the expressions are the same. For instance, NaCl and KCl have the same expression: [x][x] = x^2; therefore, you can compare the Ksp to know which on is more soluble...(highest Ksp is more soluble). However, if the expressions are not the same like NaCl and CaCl2 ...NaCl is: [x][x] = x^2 and CaCl2 is: [x][2x]^2 = 4x^3...You have to find the molar solubility of each, which is x in order to compare them. Highest molar solubility will be more soluble

 

[justhanging]

We do use the Ka to compare acidity cause its always the same form [H][A]/[HA] as suppose ksp. Look at the answer above.

 

[BestDoctorEver]

Ksp tells you the same thing when the expressions are the same. For instance, NaCl and KCl have the same expression: [x][x] = x^2; therefore, you can compare the Ksp to know which on is more soluble...(highest Ksp is more soluble). However, if the expressions are not the same like NaCl and CaCl2 ...NaCl is: [x][x] = x^2 and CaCl2 is: [x][2x]^2 = 4x^3...You have to find the molar solubility of each, which is x in order to compare them. Highest molar solubility will be more soluble

Lol... It's funny that we have to learn all these stuff to get into med school. Now that I am in med school, I forget how to calculate most of these stuff. I have seen very little physical science in med school so far...

 

[chemtopper]

We do use the Ka to compare acidity cause its always the same form [H][A]/[HA] as suppose ksp. Look at the answer above.

Ka has always a fixed formula for the dissociation of first H+ ion.
HA<----> H+ + A-
Ka= [H+][A-]/[HA] so you can compare ka to compare the concentration of H+ also.
however ksp formula changes with different compounds like AX,AX2,A2X3 etc will have different Ksp formula so in every case molar solubility can not be predicted simply by comparing the ksp.I can compare Ksp of NaCl,KCl, NH4Cl etc as they all are of AX type formula.However we can not compare Ksp of NaCl with Na3PO4 as they have different formulas.