Define conductivity, molar conductivity and cell constant. Describe how the conductivity of an electrolytic solution is measured.

1. 1 · Define conductivity. κ = 1/ρ, the conductance of a unit cube of solution; unit S m⁻¹.
2. 2 · Define molar conductivity. Λm = κ/c; unit S m² mol⁻¹.
3. 3 · Define cell constant. **G* = l/A = R × κ; unit m⁻¹**.
4. 4 · Draw the bridge. Wheatstone bridge with the conductivity cell in one arm, a variable resistance in another, an AC source and a detector. Label all parts.
5. 5 · Explain the measurement.
   • Use AC so that no electrolysis takes place
   • Balance the bridge (no current in the detector) to get R
   • Find G* using a KCl solution of known κ
   • Then κ = G*/R and Λm = κ/c
6. 6 · Link to real life. TDS meters used with home water purifiers measure conductivity to judge water quality.
7. 7 · Close in one line. So one resistance reading, with a known cell constant, gives κ and then Λm.

Scoring tip: Write the unit after every quantity. A missing or wrong unit on Λm costs half a mark.