Chemistry · Ch 5 — Electrochemistry

Variation of molar conductivity with concentration

5.3.6

Variation of molar conductivity with concentration

The variation of molar conductivity with concentration is qualitatively different for strong and weak electrolytes.

i. Strong electrolytes : The molar conductivity of a solution of a strong electrolyte increases rapidly with dilution. It approaches a limiting value for a 0.001 M or 0.0001 M solution; thereafter dilution has no effect on the molar conductivity. This maximum limiting value is the molar conductivity at zero concentration (or at infinite dilution), denoted by $\Lambda_0$ . "Zero concentration" or "infinite dilution" means the solution is so dilute that further dilution does not increase the molar conductivity.

During the nineteenth century F. Kohlrausch showed, with repeated experiments, that the molar conductivity of strong electrolytes varies linearly with the square root of concentration:

\Lambda = \Lambda_0 - a\,\sqrt{c} \qquad \text{...(5.10)}

where $a$ is a constant. For strong electrolytes, a plot of $\Lambda$ versus $\sqrt{c}$ is linear, as shown in Fig. 5.1.

Figure 5.1Graph of molar conductivity against the square root of concentration, showing the near-linear falling line of a strong electrolyte extrapolating to the limiting value at zero concentration and the steep non-linear curve of a weak electrolyte that never approaches linearity.

Fig. 5.1 — Graph of molar conductivity against the square root of concentration, showing the near-linear falling line of a strong electrolyte extrapolating to the limiting value at zero concentration and the steep non-linear curve of a weak electrolyte that never approaches linearity.

Drawn by us to help you understand the concept clearly, and verified to make sure it's accurate. For exams, practice from your textbook's own diagram.

What this figure shows. Molar conductivity (the book's wedge symbol, our $\Lambda$ ) on the y-axis against $\sqrt{c}$ on the x-axis, with no numeric ticks. The upper, near-linear curve — labelled Strong electrolyte — falls gently and extrapolates (dashed guide line) to the intercept marked ∧₀ on the y-axis: this extrapolation is how $\Lambda_0$ of a strong electrolyte is measured. The lower curve — labelled Weak electrolyte — rises steeply only at extreme dilution (leftmost part) and stays well below $\Lambda_0$ at ordinary concentrations; because it never becomes linear, the extrapol …

The molar conductivity of a strong electrolyte at zero concentration is determined by extrapolating the linear part of the $\Lambda$ versus $\sqrt{c}$ curve, as shown in Fig. 5.1. …

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