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A commercial ion-exchange resin used in water softening has the formula C8H7SO3⁻ Na⁺ and a molecular mass of 206. What is the greatest amount of Ca2+ that can be taken up by this resin, expressed as moles per gram of resin?

1. 2/309
2. 1/412
3. 1/103
4. 1/206

Correct answer: 1/412

Solution

Per gram there are 1/206 mol of resin units, each carrying one SO3- exchange site. Ca2+ is divalent and occupies two sites, so max Ca2+ uptake = (1/206)/2 = 1/412 mol per gram.

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