Page 158 - Instant notes

Physical chemistry 144

each other κ=L(l/A), with κ being the conductivity. It is the conductivity that probes the
rate of charge transfer in the solution free from variations in apparatus design. In practice
it is very difficult to construct experimental conductivity cells which exactly obey this
equation, but generally κ=LC, where C is the cell constant which can be determined by
calibration. This involves determining L for a solution of known κ, calculating C and
using it to determine κ from L for all other solutions.

Molar conductivity

The rate of charge transfer is proportional to the number of ions in the solution between
the plates, which itself will vary with the concentration, c, of dissolved electrolyte. The
molar conductivity, Λ m, is defined as:

and allows for this variation with concentration. Λ m values enable comparison of the
conductivities (or charge carrying capabilities) of an equivalent number of moles of
electrolyte both as the type and the concentration of electrolyte is varied. These
measurements allow variation in ion migrational motion in these systems to be studied.

Strong electrolytes

A strong electrolyte completely dissociates into its constituent ions. An example is
NaCl. It might be expected that Λ m would be independent of concentration for a strong
electrolyte, but in reality for dilute solutions

where κ is a constant for a particular electrolyte and c is the electrolyte concentration
(Fig. 2a).