The contribution of an electrolyte, or an ion electrolyte, is reported as the molar of a conductance.
The definition of the molar conductance is based on the following conductivity cell in which the electrodes are 1 m apart and of sufficient area that the cell holds the amount of solution that contains 1 mol of solute. The conductivity of such a cell is the mole conductance.
A of solution of concentration c, expressed in moles per litre, has a volume in litres per mole of 1/c or a volume in cubic meters of (10-3 m3 l-1)/c. a cell with this volume and electrodes separated by 1 m would be equilivalent to (10-3 m3 l-1)/c unit cells placed alongside each other. The conductivity of such a cell, which is the molar conductance, is given by:
A = 10-3 m3 l-1/c × k
This relation defines the molar conductance in terms of the specific conductance. The concept of the cell holding solution of volume (10-3 m3 l-1)/c is introduced only to suggest the definition of conductance and in practice one uses any convenient conductance cell, measures R, and calculate L = 1/R. with this datum one obtains k= (cell constant) L and finally A.
Many precise measurements of molar conductance were made by Friedrich Kohlausch and his coworkers between about 1860 and 1880.
On the basis of such data and in the absence of any satisfactory theory about the nature of conduction in these solutions, some variable empirical relations were concluded. It was recognized that for some electrolytes plotting the molar conductance of an electrolyte at a fixed temperature against the square root of the concentration led to the plots which confirmed very closely at the lower concentrations to straight lines. Such plots for new electrolytes are lead to essentially linear plots are now classed as strong electrolytes, and those which seem to approach the dilute solution limit almost tangentially are classed as weak electrolytes.
An important relation can be deduced from extrapolations of the strong electrolyte data to infinite dilution to give what are known as limiting molar of the independent migration of ions. The law is more easily stated and understandable if some later ideas are anticipated and the conductance of an electrolyte at infinite dilution is treated as being made of contributions from the individual ions of the electrolyte. Let v+ be the number of positive ions and v - the number of negative ions implied by the formula of the electrolyte.
Molar conductances ? in Ω-1 m2 mol-1 in aqueous solution at 25° C (values for c = 0obtained by extrapolation or, for HAc and NH4OH, by a combination of extrapolated values):
| c |
NaCl |
KCl |
HCl |
NaAc |
CuSO4 |
H2SO4 |
HAc |
NH4OH |
| 0.000 |
(0.012645) |
0.014986 |
0.042616 |
0.00910 |
0.02661 |
0.08592 |
0.03907 |
0.002714 |
| 0.0005 |
(0.012450) |
0.014781 |
0.042274 |
0.00892 |
0.02304 |
0.08262 |
0.00677 |
0.0047 |
| 0.001 |
0.012374 |
0.014695 |
0.042136 |
0.00885 |
0.01666 |
0.07990 |
0.00492 |
0.0034 |
| 0.010 |
0.011851 |
0.014127 |
0.041200 |
0.008376 |
0.01010 |
0.06728 |
0.00163 |
0.00113 |
| 0.100 |
0.010674 |
0.012896 |
0.039132 |
0.007280 |
0.00586 |
0.05016 |
|
0.00036 |
| 1.00 |
|
0.01119 |
0.03328 |
0.00491 |
|
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|
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