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 conductivity of a dilute solution is given as the Debye-Huckel and Onsager equation.

[pic]

where A and B are constants. By plotting Equivalent conductivity versus [pic], the molar conductivity at infinite dilution (Λ[pic]) could be determined.

The ratio between equivalent conductivity (Λ) at specific concentration and the conductivity at infinite dilution(Λ) is known as conductivity coefficient(γ).

[pic]

The relationship between equivalent conductivity (γc) and conductivity is listed as: [pic]

where ne is the electrochemical valence which is related to the number of ions (v+ or v-) and charge (z+ or z-). [pic]

Methodology:

0.1 M (200 ml) stock solution of the salts given are prepared.

From the stock solution:

0.05, 0.01, 0.005, 0.001, 0.0005 and 0.0001 solution are prepared.

The conductivity meter are calibrate using 0.10 M KCl

The conductivity of the solutions are measured beginning from the dilute solution. The electrode are rinsed before measuring. The value K in µScm-1 are recorded.

Discussion:

In this experiment, we have to determine the relationship between concentration and conductivity of various electrolytes. There are many type of chemical we used to study this relationship. These five chemical are sodium chloride, sodium sulfate, calcium chloride, potassium nitrate and manganum chloride. The molar conductivity is found to vary with the concentration. One reason for this variation is that the number of ions in the solution might not be proportional to the concentration of the electrolyte . for instance, the of ions in a solution of a weak acid depend n the concentration of the acid in a complicated way, and doubling the concentration of the acid added does not doubled the number of ions. Secondly, because ions interacts strongly with one another, the conductivity of a solution is not exactly proportional to the number of ions present. The concentration dependence of molar conductivities indicates that there...