Chemistry · Electrochemistry · NEET
Cell constant is the ratio of the distance between the two electrodes (l) to their area of cross-section (A). So G* = l/A. Because it is length divided by area (cm ÷ cm²), its unit is cm⁻¹ (or m⁻¹). It is a fixed property of that particular conductivity cell — it does not depend on the solution inside.
Both are the same thing. Conductance G = κ × (A/l) = κ / G*. Also G = 1/R. Rearranging gives G* = κ × R. So when a NEET question gives you conductivity κ and resistance R, use G* = κ × R. When it describes the cell's shape, use G* = l/A. They always give the same number.
Step 1: Find the cell constant. Fill the cell with a standard KCl solution (its κ is known from tables), measure its resistance R on a Wheatstone bridge, then G* = κ(KCl) × R. Step 2: Now put the unknown solution in the same cell, measure its resistance R. Its conductivity is κ = G* / R. That is why NCERT writes κ = cell constant / R.
We cannot measure the tiny distance l and area A of the electrodes accurately. So instead we use a solution whose conductivity is already known very precisely — KCl. Its κ at various concentrations and temperatures is listed in standard tables. Measuring its resistance lets us back-calculate G* = κ × R for that cell.
If we passed direct current through the solution, it would cause electrolysis and change the composition of the solution near the electrodes, giving a wrong resistance. Using AC (an audio-frequency oscillator, 550–5000 Hz) keeps reversing the direction, so no net electrolysis happens and the reading stays true. This is a common one-line NEET fact.
No. Conductance G = 1/R (unit siemens, S) is for the whole cell of solution. Conductivity κ = G × (l/A) = G × cell constant (unit S cm⁻¹) is for a standard 1 cm cube of solution. The cell constant is exactly the bridge between the two: κ = G × G* = G*/R.
The conductivity of a centimolar (0.01 M) 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
The molar conductivity of a 0.5 mol dm⁻³ solution of AgNO₃ with electrolytic conductivity (κ) of 5.76 × 10⁻³ S cm⁻¹ at 298 K is
Try the real previous-year questions from this chapter — each with the answer and a full solution.
cm⁻¹ (or m⁻¹). It comes from length divided by area: cm / cm² = cm⁻¹.
No. It depends only on the cell's geometry — the distance between electrodes and their area. It stays the same whichever solution you pour in, which is why one KCl calibration works for all later measurements in that cell.
κ = conductance × cell constant = G × G* = G*/R. So conductivity = cell constant / resistance.
The electrode spacing and area are very small and hard to measure accurately and reliably. Calibrating with KCl of known conductivity gives a far more accurate cell constant.
Yes. NEET regularly asks you to find cell constant (G* = κR), or to first find G* and then use it to get κ or molar conductivity Λm of another solution.
