Lesson 8, Topic 5
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First law states that the mass m, of a substance liberated or deposited during electrolysis is directly proportional to the quantity of electricity passing through it.

$$\scriptsize M \propto Q$$

But Q=It

$$\scriptsize M \propto It$$

M = ZIt

Z = electrochemical equivalent of the substances

I = current in amperes, t = time in seconds

Electrochemical equivalent of a substance is the mass of the substance deposited by one coulomb of electricity during electrolysis.

Second law states that if the same quantity of electricity is passed through different electrolytes, the mass of element liberated or deposited is directly proportional to their chemical equivalent.

Examples

1. Find the mass of silver deposited on the cathode of a silver voltameter if a current o 0.7A is passed through it for 25 minutes. (E.c.e of silver= 4.5 x 10-4 gc-1)

Solution

M = ZIT

= 4.5 x 10-4 x 0.7 x 25 x 60

=0.473g

2. What mass of copper will be deposited by a current of 0.53A in 15 minutes? If e.c.e of copper is 3.3 x 10-4gc-1

Solution

M = ZIT

= 3.3 x 10-4 x 0.53 x 15 x 60

= 0.157g

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