Answer:

Synthetic resins are used for the treatment of permanent hardness of water. Cations and anions present in water are exchanged by hydrogen and hydroxide ions of the resin. Synthetic resins are classified into cation exchange resins and anion exchange resins.

Cation exchange resins contain large organic molecules having −SO3H group. Initially, the resin is treated with NaCl which changes it from RSO3H to RSO3Na.

Hard water is then passed through the resin and Na+ ions of the resin are exchanges with Ca2+ and Mg2+ ions from hard water thereby making water soft.

In other type of cation exchange resins, the H+ ions from resin are exchanged with Na+,Mg2+ and Ca2+ ions from hard water.

In the anion exchange resins, OH− ions from resin are exchanged with Cl−,HCO3− and SO42− ions from hard water.

Hard water is passed through cation exchange resin followed by anion exchange resin. The water is demineralized and neutral in nature.

Explanation:

The hydrogen ion concentration is calcuated from pH

[H+]=10−pH=10−3M

As two H+ ions are needed for 1 calcium ion, the concentration of Ca(II) is twice the concentration of hydrogen ions.

[Ca2+]=2[H+]=2×10−3M

The molar mass of Ca is 40.

Hence, the ppm of Ca(II) =100040×2×10−3×106=80ppm 

Thus, the hardness of water in terms of Ca(II) is 80 ppm.

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