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Masses or Volumes from Electrolysis.

A unit of electrical
charge is called a coulomb.

One coulomb (symbol Q) is 6·2 x 10^{18} electrons.

One mole of electrons
is the same as 96,500 coulombs.

96,500 coulombs is called a faraday.

Electric current
(symbol I) is a flow of electrical
charge.

The rate of flow of electrical
charge is measured in amps (A).

One amp
is one coulomb per second. Q = I x t.

Example 3.

The electrolysis of molten lead bromide - PbBr_{2}

produces lead at the
cathode and bromine at the anode.

A current of 10 amps is
allowed to flow

through molten lead bromide for
5 hours.

What mass of lead is deposited at the cathode?

Method.

1) Write the half equation for the
electrolysis.

Pb^{2+}
+ 2e^{-} Pb

2 moles of electrons (2 faradays)

are required to deposit 1 mole of
lead.

2) Find how many faradays have passed
through

the lead bromide in 5
hours.

5
hours contains (5 x
60 x 60)
seconds

= 18,000
seconds.

Q =
10 x 18,000

= 180,000 coulombs.

1 faraday = 96,500 coulombs.

180,000
coulombs = 180,000 ÷
96,500
faradays

= 1·865 faradays.

3) From the
proportion in 1 above,

2 faradays are required to deposit 1 mole of
lead.

1·865 faradays will deposit
(1·865 ÷ 2)
moles of lead,

= 0·933 moles of lead.

RAM of lead = 207

mass
= 0·933 x 207

= 193 g of lead.

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