Limiting molar conductivity of an electrolyte is the sum of contributions of its individual ions.
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Kohlrausch's law states that the limiting molar conductivity of an electrolyte equals the sum of the independent contributions of its ions, each multiplied by its stoichiometric number: Lambda0 = nu+ lambda+ + nu- lambda-. Each ion contributes the same amount regardless of its partner, which is why the law is so useful. It allows the limiting conductivity of a weak electrolyte, such as acetic acid, to be calculated from strong electrolyte data. Applying it requires careful bookkeeping of ion contributions. It also supports determination of the degree of ionization of weak electrolytes.
Key ideas: At infinite dilution, each ion contributes independently: Lambda0 = nu+ lambda0+ + nu- lambda0-. Use it to find Lambda0 of weak electrolytes: Lambda0(CH3COOH) = Lambda0(CH3COONa) + Lambda0(HCl) - Lambda0(NaCl), about 390.7 S cm2/mol. Then alpha = Lambda_m/Lambda0 and Ka = c alpha^2/(1 - alpha).
Common mistakes and connections: Mistakes: ignoring stoichiometric coefficients (CaCl2 needs 2 x lambda of Cl-), and using it at finite concentration, where ions interact. The key is that ions move independently only at infinite dilution. Ion values such as H+ 349.8 and OH- 198.6 S cm2/mol (25 C) are large owing to proton hopping.
Kohlrausch: Lambda0 (CH3COOH) = 426 + 91 - 126.5 = 390.5, found by adding strong electrolyte values because each ion contributes independently. The interactive lesson shows this as a live animation or chart.
Lambda0(CH3COOH) = Lambda0(HCl) + Lambda0(NaAc) - Lambda0(NaCl), where NaAc is sodium acetate.
Limiting molar conductivity of an electrolyte is the sum of contributions of its individual ions.
Kohlrausch's Law is a advanced topic in Acids, Bases & Electrochemistry. It builds on beginner and intermediate topics, so review those first if needed.
Study these earlier topics in Acids, Bases & Electrochemistry first: Galvanization, Conductance and Conductivity, Molar Conductivity.
The next topics in Acids, Bases & Electrochemistry are Strong and Weak Electrolytes and Ionic Mobility.
Yes. The interactive lesson includes a self-check question on Kohlrausch's Law, and the Acids, Bases & Electrochemistry quiz draws on every topic in this area.
Kohlrausch's Law is topic 82 of 105 in Acids, Bases & Electrochemistry.