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Absolute alcohol (100% ethanol) is obtained by distilling rectified spirit in the presence of which drying agent?

1. Sodium
2. Calcium chloride
3. Magnesium
4. Magnesium ethoxide

Correct answer: Magnesium ethoxide

Solution

Rectified spirit (~95% ethanol) is dried to absolute alcohol by treatment with magnesium, which forms magnesium ethoxide; the magnesium ethoxide reacts with the residual water [Mg(OC2H5)2 + H2O -> Mg(OH)2 + 2C2H5OH], removing it. CaCl2 cannot be used as it forms a complex with ethanol.

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