Chemistry · Electrochemistry · NEET
Conductivity is the conductance of a solution held between two electrodes that are 1 cm apart and have 1 cm² area. In short, it is the conductance of a 1 cm cube of the solution. Its symbol is κ (kappa) and its unit is S cm⁻¹ (or S m⁻¹). It is the inverse of resistivity. It does NOT tell you how many ions are present, only how well that fixed volume conducts.
Molar conductivity is the conducting power of ALL the ions that come from one mole of electrolyte dissolved in the solution. Symbol is Λm (lambda). It divides out the amount of substance, so it is a fairer way to compare different electrolytes. Its unit is S cm² mol⁻¹.
Conductivity depends on how many ions are packed into that 1 cm cube. A concentrated solution has more ions per cube, so it can look more conductive just because it is crowded, not because its ions are better. Molar conductivity fixes this by counting per mole of electrolyte, so you compare the true conducting ability. That is WHY NEET asks about Λm and not just κ.
Λm = κ / c. But if c is given in mol L⁻¹ (mol dm⁻³) and κ in S cm⁻¹, you must convert litre to cm³ (1 L = 1000 cm³), so the working formula is Λm = (κ × 1000) / c. This 1000 factor is the most common NEET trap. Answer comes out in S cm² mol⁻¹.
On dilution, conductivity (κ) DECREASES because there are fewer ions in each 1 cm cube. But molar conductivity (Λm) INCREASES because the same one mole of ions now spreads out and moves more freely. Remember: they move in OPPOSITE directions with dilution. This opposite behaviour is a favourite NEET question.
Conductivity κ has units S cm⁻¹ (or S m⁻¹). Molar conductivity Λm has units S cm² mol⁻¹ (or S m² mol⁻¹). The extra 'cm²' and 'mol⁻¹' appear because we divide κ by concentration (mol per volume), which brings in volume per mole. Mixing up these units loses easy marks.
The molar conductivity of a 0.5 mol dm⁻³ solution of AgNO₃ with electrolytic conductivity of 5.76 × 10⁻³ S cm⁻¹ at 298 K is
The conductivity of centimolar solution of KCl at 25 °C is 0.0210 ohm⁻¹ cm⁻¹ and the resistance of the cell containing the solution at 25 °C is 60 ohm. The value of cell constant is
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Usually yes in the common units, because Λm = κ × 1000 / c and dividing by a small concentration makes the number bigger. But they are different physical quantities with different units, so you should compare them by meaning, not just size.
κ is the Greek letter kappa. It stands for conductivity (also called specific conductance), the conductance of a 1 cm cube of solution. IUPAC prefers the word 'conductivity' over 'specific conductance'.
Λm° is the limiting molar conductivity, the value of molar conductivity when the solution is so dilute that ions no longer interfere with each other. It is found by extrapolation for strong electrolytes and by Kohlrausch's law for weak electrolytes.
A conductivity meter reads conductance or conductivity (κ) of the solution once the cell constant is set. You then calculate molar conductivity from κ and the known concentration.
It is used to compare strong versus weak electrolytes, apply Kohlrausch's law, and find degree of dissociation (α = Λm / Λm°) of weak acids. These appear as calculation questions almost every year.
