Note: I is the current in amps,
t is the time in seconds,
Q is the number of coulombs of electrical charge that have passed through the electrolytic cell. This charge can be related to the charge on one mole of electrons.
1 mole of electrons has a charge of 96,500 coulombs = 1 Faraday
You will need to take into account how many F of charge is needed to deposit 1 mole of the substance in question. So you will need the following electrode equation:
Al3+ + 3e→Al
so 1 mole of Ca needs 2 F but 1 mole of Al needs 3F
Suppose a current of
amps is passed through molten magnesium chloride for 1 hour. What mass of magnesium will be formed at the cathode?
Follow this scheme to answer the question:
1. Calculate Q
Q = I x t
=2.5 ×10 2×60 ×60
=9 ×10 5coulombs
2. Calculate the number of moles of
electrons passed (Faradays)
F = Q / 96500
=9 ×10 5/96,500
=93.3F(to 3 s.f.)
3. Consider the equation
Mg2+ + 2e→Mg
4. So no. of moles of substance formed
= moles of substance / no of moles of e in the equation.
5. Mass of product = no of moles
Hint: keep all the numbers in the calculator and only round off to three significant figures (3 s.f.) at step 5.