PHYSICAL CHEMISTRY IN BRIEF

CHAP. 11: ELECTROCHEMISTRY [CONTENTS] 355

When we are interested in finding the total massmof substance Kν+Aν−used up at the
electrodes (in the form of ions), we can use Faraday’s law in the form

m=

M I τ

z+ν+F

=

M I τ

z−ν−F

, (11.9)

whereM is the molar mass of the substance Kν+Aν−.
Faraday’s law can be also used when the products of oxidation or reduction react at the
electrode further on. In such cases we first have to write the overall reaction and then use the
relation

mi=

MiI τ

zeF

νi, (11.10)

whereziis the number of electrons in the reaction andνiis the stoichiometric coefficient of
substancei.

Example

An aqueous solution of aluminium sulfate was electrolyzed for one hour by an electric current

of 0.1 A. The molar mass of aluminium isMAl= 27 g mol−^1 and the molar mass of aluminium

sulfate isM = 343 g mol−^1.

a) Calculate the mass of aluminium discharged at the cathode.

b) Calculate the mass of aluminium sulfate that dissociated at the electrodes.

c) Calculate the mass of oxygen discharged at the anode.