A solution contains `0.1M` each of `CaCI_(2)` and `SrCI_(2). A 0.005M` solution of `SO_(4)^(2-)` is slowly added to the given solution.
a. Which substance beings to precipiate first?
b. If `H_(2)SO_(4)` is continuosult added, determine when will other salt be precipitated?
c. When second salt starts to precipitate, find the concentration of cation of first salt. Assume that `CaCI_(2)` and `SrCI_(2)` are `100%`ionised and volume of the solution remains constant.
`K_(sp) of SrSO_(4) = 3.2 xx 10^(-7)` and `K_(sp)of CaSO_(4) = 1.3 xx 10^(-4)`

Explanation

The solution `0.01M Ca^(2+)` ion and `0.01M Sr^(2+)` ions `(100%` ionisation). Now `SO_(4)^(2-)` ions are added gradually and slowly.
a. First calculate the minimum `[SO_(4)^(2-)]` required to start precipitation of `CaSO_(4)` and `SrSO_(4)`.
`[SO_(4)^(2-)_("Min. for CaSO"_(4)) = [K_(sp)[CaSO_(4)])/([Ca^(2+)])`
`= (1.3 xx 10^(-4))/(0.01) = 1.3 xx 10^(-5)M`
Since `[SO_(4)^(2-)]_("Min.For SrSO"_(4)) = (K_(sp)[SrSO_(4)])/([Sr^(2+)])`
`= (3.2 xx 10^(-7))/(0.01) - 3.2 xx 10^(-5)M`
Since `[SO_(4)^(2-)]` for `SrSO_(4)` is much lower that required for `CaSO_(4)`, so `SrSO_(4)` will precipitate first.
b. Now, if `SO_(4)^(2-)` ions are continuously added, at some instant, its concentration will become equal to that minimum required for precipitating out `Ca^(2+)` ions. Hence `CaSO_(4)` will start precipitating if,
`[SO_(4)^(2-)] = 1.3 xx 10^(-2)M`.
c. Now, find the concentration of `Sr^(2+)` ions when
`[SO_(4)^(2-)] = 1.3 xx 10^(-2)M` in the solution.
`[Sr^(2+)]' = (K_(sp)SrSO_(4))/([SO_(4)^(2-)]) = (3.2 xx 10^(-7))/(1.3 xx 10^(-2)) = 2.46 xx 10^(-5)M`
`[2.46 xx 10^(-5)M lt lt lt 0.01M]`
Note that when `CaSO_(4)` starts to precipitate, `SrSO_(4)` is almost precipitated out as the value of `Sr^(2+)` is very very low.
Note: The above illustration repr4esents a case of selective precipitaiton (or separation of matals cations) by adding a common anion. Note that there is a vast difference in the `K_(sp)` value of two suplphates, and hence in the minimum value of sulphate ion for precipiating out the two metals ions.