Question

State Kohlrausch law of independent migration of ions. Why does the conductivity of a solution decrease with dilution?

Why does the conductivity of a solution decrease with dilution ?

The resistance and conductivity of a conductivity cell containing 0·001 M KCl solution at 298 K are 1200 Ω and 1·5 × 10⁻⁴ S cm⁻¹. Calculate its cell constant and molar conductivity.

Read the passage given below and answer the questions that follow : Oxidation-reduction reactions are commonly known as redox reactions. They involve transfer of electrons from one species to another. In a spontaneous reaction, energy is released which can be used to do useful work. The reaction is split into two half reactions. Two different containers are used and a wire is used to drive the electrons from one side to the other and a Voltaic/Galvanic cell is created. It is an electrochemical cell that uses spontaneous redox reactions to generate electricity. A salt bridge also connects to the half cells. The reading of the voltmeter gives the cell voltage or cell potential or electromotive force. If E°cell is positive the reaction is spontaneous and if it is negative the reaction is non-spontaneous and is referred to as electrolytic cell. Electrolysis refers to the decomposition of a substance by an electric current. One mole of electric charge when passed through a cell will discharge half a mole of a divalent metal ion such as Cu²⁺. This was first formulated by Faraday in the form of laws of electrolysis. The conductance of material is the property of materials due to which a material allows the flow of ions through itself and thus conducts electricity. Conductivity is represented by k and it depends upon nature and concentration of electrolyte, temperature etc. A more common term molar conductivity of a solution at a given concentration is conductance of the volume of solution containing one mole of electrolyte kept between two electrodes with the unit area of cross-section and distance of unit length. Limiting molar conductivity of weak electrolytes cannot be obtained graphically.

Rahul set-up an experiment to find resistance of aqueous KCl solution for different concentrations at 298 K using a conductivity cell connected to a Wheatstone bridge. He fed the Wheatstone bridge with a.c. power in the audio frequency range 550 to 5000 cycles per second. Once the resistance was calculated from null point he also calculated the conductivity K and molar conductivity Λm and recorded his readings in tabular form. S.No. 1: Conc.(M) 1.00, k S cm–1 111.3 × 10–3, Λm S cm2 mol–1 111.3. S.No. 2: Conc.(M) 0.10, k S cm–1 12.9 × 10–3, Λm S cm2 mol–1 129.0. S.No. 3: Conc.(M) 0.01, k S cm–1 1.41 × 10–3, Λm S cm2 mol–1 141.0.