A sample of hard water contains `0.05mol` of `CaC1_(2)`, per litre, 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)` is `2.4 xx 10^(-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)` from a solution having `[Ca^(2+)] = 0.005M`
The `[0.005] [a] = 2.4 xx 10^(-5)`
`:. A = (2.4 xx 10^(-5))/(0.005)`
`[SO_(4)^(2-)] = 4.8 xx 10^(-3) mol L^(-1)`