Question

Ksp=2.4x10^-5 for calcium sulfate. What is the molar solubility of calcium sulfate in pure water? What is the mass solubility of calcium sulfate in pure water, expressed in g/L?

Ksp=2.4x10^-5 for calcium sulfate.

What is the molar solubility of calcium sulfate in pure water?

What is the mass solubility of calcium sulfate in pure water,
expressed in g/L?

Answer 1

(a)

`sf(4.9xx10^(-3)color(white)(x)"mol/l")`

(b)

`sf(0.67color(white)(x)"g/l")`

Explanation:

(a)

Calcium sulfate is sparingly soluble. In a saturated solution the aqueous ions are in equilibrium with the insoluble salt:

`sf(CaSO_(4(s))rightleftharpoonsCa_((aq))^(2+)+SO_(4(aq))^(2-))`

For which

`sf(K_(sp)=[Ca_((aq))^(2+)][SO_(4(aq))^(2-)]=2.4xx10^(-5)color(white)(x)"mol"^2."l"^-2)`

The solubilty `sf(s)` of the salt is equal to `sf([Ca_((aq))^(2+)])`.

Since `sf([Ca_((aq))^(2+)]=[SO_(4(aq))^(2-)])` we can write:

`sf(K_(sp)=s^(2)=2.4xx10^(-5))`

`:.` `sf(s=sqrt(2.4xx10^(-5))=4.9xx10^(-3)color(white)(x)"mol/l")`

This is the molar solubility.

(b)

To find the mass solubility we multiply by the mass of 1 mole. I will assume we are using the anhydrous salt `sf(CaSO_4)` for which the `sf(M_r=136.1)`

`:.` Mass solubility = `sf(4.9xx10^(-3)xx136.1=0.67color(white)(x)"g/l")`