A sample of hard water contains 0.005 mole of CaCl2 per litre. What is the minimum concentration of Na2SO4 which must be exceeded for removing calcium ions from their water solution? (The solubility product of CaSO4 is 2.4×10−5.)
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Solution
The solubility product of CaSO4,Ksp=2.4×10−5
∴[Ca2+]=0.005M
⇒Ksp=[Ca2+][SO2−4]
⇒[SO2−4]=2.4×10−50.005=4.8×10−3M
∴ The minimum concentration of Na2SO4 will be 4.8×10−3M .