PHYSICAL CHEMISTRY IN BRIEF

CHAP. 11: ELECTROCHEMISTRY [CONTENTS] 363

Solution

The first step is to determine the conductivity from relation (11.25)

κ=

541

1440

= 0. 3757 S m−^1.

Then we use relation (11.26) to calculate the molar conductivity—we make sure that we converted

the molar concentration value to the SI system

Λ =

0. 3757

0. 02 × 103

= 187. 8 × 10 −^4 S m^2 mol−^1.

Molar conductivity depends on concentration, temperature and pressure. Its concentration
dependence is dealt with in section11.3.7. The value of Λ usually increases with temperature.
The pressure dependence is very low and it is usually neglected.

11.3.4 Kohlrausch’s law of independent migration of ions.

Themolar conductivity at infinite dilutionΛ∞is defined by the relation

Λ∞= lim

c→ 0

Λ. (11.27)

It applies that
Λ∞=ν+Λ+∞+ν−Λ−∞, (11.28)

where Λ+∞is themolar conductivity of the cation at infinite dilutionand Λ−∞is themolar
conductivity of the anion at infinite dilution. Relation (11.28) is calledKohlrausch’s
law of the independent migration of ions. It expresses the fact that ions in an infinitely
diluted solution do not influence each other, see11.1.6.
The molar conductivity of ions at infinite solution can be found in tables. Their values
together with relation (11.28) are used to calculate the molar conductivity of electrolytes at
infinite dilution.