Solubility

[Opejay]

Can someone help me out with this please.

Ag2SO4 has a Ksp value of 1.2E-5 and BaSO4 has a Ksp value of 1.5E-9

I thought BaSO4 should be less soluble than AgSO4 because it has a lower Ksp value but EK says otherwise.

Please help me understand this

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[indianjatt]

Always remember that solubility depends on the molar solubility x, and not the Ksp. The Ksp value may be deceiving. Sometimes a lower Ksp for a compound is actually the more soluble one.

In this case, the molar solubility for Silver sulfate is 4x^3=12 x 10^-6; x = 1..... x 10^-2 (around there)
The molar solubility for Barium sulfate is x^2 = 15E-10; x = 4 x 10^-5 (approx). This we can see that the molar solubility for Ag2SO4 > BaSO4; x = 10^-2 > x = 10^-5

 

[Temperature101]

Can someone help me out with this please.

Ag2SO4 has a Ksp value of 1.2E-5 and BaSO4 has a Ksp value of 1.5E-9

I thought BaSO4 should be less soluble than AgSO4 because it has a lower Ksp value but EK says otherwise.

Please help me understand this

Like the previous poster said, solubility always depend on the molar solubility. But if both compounds have the same Ksp expressions such Ag2SO4 and Na2SO4, you can compare their Ksp to find which one is more soluble.

 

[sciencebooks]

Always remember that solubility depends on the molar solubility x, and not the Ksp. The Ksp value may be deceiving. Sometimes a lower Ksp for a compound is actually the more soluble one.

In this case, the molar solubility for Silver sulfate is 4x^3=12 x 10^-6; x = 1..... x 10^-2 (around there)
The molar solubility for Barium sulfate is x^2 = 15E-10; x = 4 x 10^-5 (approx). This we can see that the molar solubility for Ag2SO4 > BaSO4; x = 10^-2 > x = 10^-5

Sorry if this is a stupid question, but where did the 4 come from above?

 

[Raiden2012]

Ag2SO4 ---- (Ag)2 (SO4)
Let amount to dissolve be x
so there will be 2 Ag and 1 SO4
(2x)^2 (x) = 4x^3

 

[sciencebooks]

Ag2SO4 ---- (Ag)2 (SO4)
Let amount to dissolve be x
so there will be 2 Ag and 1 SO4
(2x)^2 (x) = 4x^3

I don't know why I'm feeling so confused on this. I thought you would write that Keq = [Ag]^2 * [SO4] = x^2 * x = x^3. Like that the stoichiometric coefficient here become both the exponents? But why is it also put in as a multiple too? Lol, I feel really oddly confused here...

 

[Raiden2012]

I don't know why I'm feeling so confused on this. I thought you would write that Keq = [Ag]^2 * [SO4] = x^2 * x = x^3. Like that the stoichiometric coefficient here become both the exponents? But why is it also put in as a multiple too? Lol, I feel really oddly confused here...

Ksp = [Ag]^2 * [SO4]
For x amount of Ag2SO4,
there will be 2x amount of Ag and x amount of SO4
This is SEPARATE from the Ksp
substitute the x values back in the Ksp equation.

It is easy to confuse the exponent and the multiple. Hope this makes sense

 

[member232]

Ag2SO4 --> [2 Ag+] + [SO4 2-]

The silver sulfate molecule dissociates into two silver ions and one sulfate ion. The Keq uses Molar concentrations raised to the exponent of their coefficient. Hence, say you had a 1 molar solution of silver sulfate. That would dissociate int TWO moles of silver ions and ONE mole of sulfate ions.

Keq becomes [concentration of silver]^2 x [concentration of sulfate] ^1

= [2x]^2 [x] or 4x^3

 

[sciencebooks]

Ksp = [Ag]^2 * [SO4]
For x amount of Ag2SO4,
there will be 2x amount of Ag and x amount of SO4
This is SEPARATE from the Ksp
substitute the x values back in the Ksp equation.

It is easy to confuse the exponent and the multiple. Hope this makes sense

Ah, that makes sense. I don't remember ever have a problem with these problems so I don't know why this just tripped me up so much. Is it just in cases of complete dissociation where the exponent and the multiple are the same?

 

[Raiden2012]

It does not have to be completely dissociated. It is a solubility product after all. Even in cases where the solubility of the product is low like barium sulphate, you can still apply this method. Because regardless, when an x amount of the solute is dissolved (even if it is very small), there will be x amount of Barium ions and x amount of Sulphate ions that go into solution

 

[Opejay]

Always remember that solubility depends on the molar solubility x, and not the Ksp. The Ksp value may be deceiving. Sometimes a lower Ksp for a compound is actually the more soluble one.

In this case, the molar solubility for Silver sulfate is 4x^3=12 x 10^-6; x = 1..... x 10^-2 (around there)
The molar solubility for Barium sulfate is x^2 = 15E-10; x = 4 x 10^-5 (approx). This we can see that the molar solubility for Ag2SO4 > BaSO4; x = 10^-2 > x = 10^-5

Thanks!

 

[Class1P]

If the number of ions your molecule dissociates into are the same then you can compare Ksp values. But i they are different like the ones you states (one becomes 3 ions and the other 2) then you have to calculate molar solubilities to compare them.