A sample of hard water contasins `0.005` mole of `CaCI_(2)` per liter. What is the minimum concentration of `Na_(2)SO_(4)` which must be added for removing `Ca^(2+)` ions from this water sample ? `(K_(SP)for CaSO_(4)=2.4xx10^(-5) at 25^(@)C)`

Explanation

For `CaSO_(4)hArr Ca^(2+)+SO_(4)^(2-)`
`[Ca^(2+)][SO_(4)^(2-)]=K_(SP)`
Let `[SO_(4)^(2-)]=a`, just sufficient to precipitate `CaSO_(4)` form a solution having `[Ca^(2+)]=0.005M`
Then, `[0.005][a]=2.4xx10^(-5)`
`:. A=(2.4xx10^(-5))/(0.005)`
`[SO_(4)^(2-)]=4.8xx10^(-3)mol litre^(-1)`