Title: Conductivity of Strong Electrolytes

Date: 16 August 2005

Objectives: ▪ To determine the relationship between the concentration and conductivity of various electrolytes ▪ To determine the conductivity at infinite dilution ▪ To determine the activity coefficients

Theory:

The resistance, R of a conductor with a similar cross section is proportional to the length (l) and inverse to the cross section area(A), therefore;

[pic]

where ρ- resistance k=1/ρ-conductivity L=1/R-conductance

For an electrolyte,

[pic] Unit:ohm-1cm-1 or S cm-1 (S=siemens)

To measured the conductivity of a solution, a cell dimension such as A and l must be known. Usually th cell is standardized with a solution of a known conductivity. The ratio of a measured conductivity toward the standard solution is the ratio of length(l) towards cross section (A) of the cell. This ratio cell is call the cell constant. Use potassium chloride with a known conductivity as a standard solution.

The conductivity measurements are not appropriate to compare between two electrolyte solutions because of the high dependant towards solution concentration. The molar conductivity (Λm) measurements would be more appropriate. Λm could be determined from the conductivity value:

[pic]

where C is the electrolyte concentration In mol/L. the unit is S cm2mol-1

If the dependant of conductivity towards concentration were to be studied, we would observe that the conductivity increases along with solution concentration due to the increase in the number of ions. However the relationship is not linear. So the equivalent conductivity is not constant for an electrolyte at different concentrations. The equivalent conductivity approaches a value called the molar conductivity at infinite dilution (Λω) as the electrolyte becomes more dilute.

For a strong electrolyte, the equivalent

References: Laboratory Manual Physical Chemistry,CHM 3010 Department of Chemistry, Faculty of Science, UPM Chemistry Matter and Its Changes, Fourth Edition Brady, Russell and Senese. Atkins’ Physical Chemistry, 7th Edition By Peter Atkins and Julio De Paula.