Conductivity and Molar Conductivity

Identify whether the question asks for kappa, conductance, cell constant, or Lambda m. Convert concentration to the unit required by the formula. Substitute with the correct 1000 factor if using mol L-1 and S cm-1. For Kohlrausch's law, write ions with coefficients and add their limiting ionic contributions.

Useful visuals include a conductivity cell labelled with electrodes, electrolyte, cell constant, and conductance meter, plus a graph of molar conductivity versus square root of concentration for strong and weak electrolytes.

A common wrong answer is reporting 0.12 S cm2 mol-1 instead of 120 S cm2 mol-1 because the 1000 conversion factor was omitted.

Conductivity is the conductance of an electrolyte solution of unit length and unit cross-sectional area, while molar conductivity is the conductance due to all ions produced by one mole of electrolyte in solution.

Conductivity is the conductance of an electrolyte solution of unit length and unit cross-sectional area, while molar conductivity is the conductance due to all ions produced by one mole of electrolyte in solution. Key formula: Lambda m = kappa x 1000 / c when kappa is in S cm-1 and c is in mol L-1. Unit of kappa: S cm-1 or S m-1. Unit of Lambda m: S cm2 mol-1 or S m2 mol-1. Kohlrausch's law: Lambda degree m of electrolyte equals the sum of limiting ionic molar conductivities of its ions with stoichiometric coefficients. If conductivity kappa = 0.012 S cm-1 and concentration c = 0.10 mol L-1, molar conductivity Lambda m = kappa x 1000 / c = 0.012 x 1000 / 0.10 = 120 S cm2 mol-1.

Conductivity depends on the number of ions per unit volume and their mobility. On dilution, conductivity usually decreases because the number of ions per unit volume decreases. Molar conductivity generally increases on dilution because ions move more freely and, for weak electrolytes, ionisation increases. Kohlrausch's law states that at infinite dilution, each ion contributes independently to the molar conductivity of the electrolyte. Key formula: Lambda m = kappa x 1000 / c when kappa is in S cm-1 and c is in mol L-1. Unit of kappa: S cm-1 or S m-1. Unit of Lambda m: S cm2 mol-1 or S m2 mol-1. Kohlrausch's law: Lambda degree m of electrolyte equals the sum of limiting ionic molar conductivities of its ions with stoichiometric coefficients. For NaCl, if kappa = 1.26 x 10-2 S cm-1 and c = 0.10 mol L-1, Lambda m = (1.26 x 10-2 x 1000)/0.10 = 126 S cm2 mol-1. The answer should have molar conductivity units, not conductivity units. Questions commonly ask calculation of molar conductivity, comparison of strong and weak electrolyte graphs, application of Kohlrausch's law to weak electrolytes, or explanation of dilution effects. A common wrong answer is reporting 0.12 S cm2 mol-1 instead of 120 S cm2 mol-1 because the 1000 conversion factor was omitted.