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The same electric current is passed in series through a silver coulometer and a water coulometer. At the cathode of the silver coulometer, the mass increased by 0.108 g. What volume of oxygen, measured at STP, is liberated in the water coulometer?

1. 56.75 cm³
2. 567.5 cm³
3. 5.675 cm³
4. 113.5 cm³

Correct answer: 5.675 cm³

Solution

Equivalents of silver deposited equal equivalents of oxygen evolved. 0.108 g Ag = 0.001 equivalent. Oxygen's equivalent volume at STP is 5600 cm³ (22400/4). So volume = 0.001 * 5600 = 5.6 cm³, i.e. about 5.675 cm³.

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