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Variation of Conductivity & Molar Conductivity with Concentration | Chemistry Class 12 - NEET PDF Download

What is Specific Conductivity?
Specific conductivity or conductivity of an electrolytic solution at any given concentration is the conductance of unit volume of solution. It is the conductance when kept between two platinum electrodes with a unit area of cross-section.
The electrodes are at a distance of unit length.
Conductivity decreases with a decrease in concentration as the number of ions per unit volume that carry the current in a solution decrease on dilution.
The molar conductivity of a solution at a given concentration is the conductance of volume V of the solution containing one mole of electrolyte kept between two electrodes with an area of cross section A and distance of unit length.

Ʌm = К/c

Here, c = concentration in moles per volume, К = specific conductivity and Ʌm = molar conductivity. As the solution contains only one mole of electrolyte, the above equation can be modified as:

Ʌm = КV

Change in Molar Conductivity:

Molar conductivity increases with a decrease in concentration. This happens because the total volume, V, of the solution containing one mole of electrolyte also increases. Upon dilution, the concentration decreases.
Furthermore, when the concentration approaches zero, the molar conductivity of the solution is known as limiting molar conductivity, Ë°m. Variation of molar conductivity with concentration is different for both, strong and weak electrolytes.

Variation of Molar Conductivity with ConcentrationVariation of Molar Conductivity with Concentration

Variation of Molar Conductivity with Concentration of Strong Electrolytes:
For strong electrolytes, the molar conductivity increases slowly with the dilution. The plot of the molar conductivity and c1/2 is a straight line having y-intercept equal to Ë°m. The value of limiting molar conductivity, Ë°m can be determined from the graph or with the help of Kohlrausch law.

For strong electrolytes, the molar conductivity increases slowly with the dilution. Thus, the plot of the molar conductivity and c1/2 is a straight line having y-intercept equal to Ë°m. The general equation for the plot is:

Ʌm = Ë°m − Ac1/2
Where −A is a constant equal to the slope of the line. Furthermore, the value of “A” for a given solvent depends on the type of electrolyte at a particular temperature. Hence, it differs from solution to solution.

Variation of Molar Conductivity with Concentration for Weak Electrolytes:

For weak electrolytes, the graph plotted between molar conductivity and c1/2 (where c is the concentration) is not a straight line. This is because weak electrolytes have lower molar conductivities and lower degree of dissociation at higher concentrations which increases steeply at lower concentrations. Hence, we use the Kohlrausch law of independent migration of ions for determining to limit molar conductivity, Ëm° of weak electrolytes.

Solved Example
Ques: How does concentration of a solution affect its specific conductivity?
Ans:
Specific Conductivity decreases with a decrease in concentration. Since the number of ions per unit volume that carry current in a solution decrease on dilution. Hence, concentration and conductivity are directly proportional to each other.

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FAQs on Variation of Conductivity & Molar Conductivity with Concentration - Chemistry Class 12 - NEET

1. What is conductivity and how does it vary with concentration?
Ans. Conductivity is the measure of a material's ability to conduct electric current. It is directly related to the concentration of ions in a solution. As the concentration of ions increases, the conductivity of the solution also increases. This is because ions are responsible for carrying the electric charge through the solution, and a higher concentration of ions allows for more efficient charge transfer.
2. What is molar conductivity and how is it related to conductivity and concentration?
Ans. Molar conductivity is a measure of the conductivity of a solution per unit concentration of the solute. It is calculated by dividing the conductivity of the solution by the concentration of the solute. Molar conductivity takes into account the effect of concentration on conductivity, allowing for better comparison between solutions of different concentrations. As the concentration of the solute increases, the molar conductivity also increases, indicating a higher ability of the solute to conduct electricity.
3. Why does the conductivity increase with concentration?
Ans. The conductivity of a solution increases with concentration due to the increased number of ions present. In a solution, ions carry the electric charge and facilitate the flow of electricity. When the concentration of ions is higher, there are more charged particles available to conduct the electric current. This leads to an increase in conductivity as more ions contribute to the overall conductivity of the solution.
4. Is there a limit to how conductivity and molar conductivity increase with concentration?
Ans. Yes, there is a limit to how conductivity and molar conductivity increase with concentration. This is because at high concentrations, ions can start to interact with each other and form ion pairs or complexes. These interactions reduce the number of free ions available for conduction, resulting in a decrease in conductivity. This phenomenon is known as the limiting molar conductivity or the maximum conductivity attainable at infinite dilution.
5. How can conductivity and molar conductivity be used to determine the concentration of an unknown solution?
Ans. Conductivity and molar conductivity can be used in conjunction with calibration curves or mathematical models to determine the concentration of an unknown solution. By measuring the conductivity or molar conductivity of the unknown solution and comparing it to a set of known standards, the concentration can be determined. This is based on the principle that there is a direct relationship between conductivity or molar conductivity and concentration.
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