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प्रश्न
Discuss the variation of conductivity and molar conductivity with concentration.
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उत्तर
Both conductivity and molar conductivity change with the change in concentration of electrolyte. Conductivity always decreases with decreasing the concentration of both weak and strong electrolytes. This can be explained by the fact that with dilution, the number of ions carrying electric current per unit volume decreases. The conductivity of a solution at any concentration is the conductivity of a unit volume of the solution placed between two platinum electrodes having a unit cross-sectional area and situated at a unit distance from each other.
This is clear from the following equation:
C = `(kappa A)/l` = κ ...(Both A and I are in appropriate units m or cm)
The molar conductivity of a solution at a given concentration is the conductivity of volume (V) of the solution containing one mole of electrolyte dissolved in it and placed between two electrodes of cross-sectional area (A), located at a unit distance from each other. So,
Plot of c1/2 versus molar conductivity for potassium chloride (a strong electrolyte) in aqueous solution.
∧m = `(kappa A)/l` = κ
Since l = 1 and A = V (volume in which one mole of electrolyte is dissolved.)
∧m = κ V
Molar conductivity increases with decreasing concentration. This is because the total volume (V) in which one mole of electrolyte is present also increases. It has been found that on dilution of the solution, the increase in volume is much more than the decrease in κ.
Strong Electrolytes: For strong electrolytes, the value of ∧m increases gradually with dilution, and it can be represented by the following equation:
∧m = `∧_m^0 - Ac^(1//2)`
It can be seen that if ∧m is plotted against c1/2, we get a straight line with intercept A and slope equal to ‘A’. The value of constant ‘A’ at a given solvent and temperature depends on the type of electrolyte, i.e., on the charges of the cation and anion produced on dissociation of the electrolyte in solution. Thus, NaCl, CaCl2, MgSO4 are known as 1-1, 2-1 and 2-2 electrolytes, respectively. The value of ‘A’ is the same for all electrolytes of the same type.
संबंधित प्रश्न
Why does the conductivity of a solution decrease with dilution?
Write mathematical expression of molar conductivity of the given solution at infinite dilution.
Molar conductivity denoted by the symbol Λm is related to the conductivity of the solution by the equation (k is the conductivity and c is the concentration).
Which of the statements about solutions of electrolytes is not correct?
Molar conductivity of ionic solution depends on:
(i) temperature.
(ii) distance between electrodes.
(iii) concentration of electrolytes in solution.
(iv) surface area of electrodes.
Solutions of two electrolytes ‘A’ and ‘B’ are diluted. The Λm of ‘B’ increases 1.5 times while that of A increases 25 times. Which of the two is a strong electrolyte? Justify your answer.
Match the items of Column I and Column II on the basis of data given below:
`E_("F"_2//"F"^-)^Θ` = 2.87 V, `"E"_(("Li"^(+))//("Li"^-))^Θ` = − 3.5V, `"E"_(("Au"^(3+))//("Au"))^Θ` = 1.4 V, `"E"_(("Br"_(2))//("Br"^-))^Θ` = 1.09 V
| Column I | Column II |
| (i) F2 | (a) metal is the strongest reducing agent |
| (ii) Li | (b) metal ion which is the weakest oxidising agent |
| (iii) Au3+ | (c) non metal which is the best oxidising agent |
| (iv) Br– | (d) unreactive metal |
| (v) Au | (e) anion that can be oxidised by Au3+ |
| (vi) Li+ | (f) anion which is the weakest reducing agent |
| (vii) F– | (g) metal ion which is an oxidising agent |
Assertion: `"E"_("Ag"^+ //"Ag")` increases with increase in concentration of Ag+ ions.
Reason: `"E"_("Ag"^+ //"Ag")` has a positive value.
Consider figure and answer the question to given below.
How will the concentration of Zn2+ ions and Ag+ ions be affected after the cell becomes ‘dead’?
The limiting molar conductivities Λ° for NaCl, KBr and KCl are 126, 152 and 150 S cm2 mol–1 respectively. The limiting molar conductivity Λ° for NaBr is ______.
The molar conductance of NaCl, HCl and CH3COONa at infinite dilution are 126.45, 426.16 and 91.0 S cm2 mol−1 respectively. The molar conductance of CH3COOH at infinite dilution is ______.
Choose the right option for your answer.
The molar conductivity of CH3COOH at infinite dilution is 390 Scm2/mol. Using the graph and given information, the molar conductivity of CH3COOK will be:
Given below are two statements:
Statements I: The limiting molar conductivity of KCl (strong electrolyte) is higher compared to that of CH3COOH (weak electrolyte).
Statement II: Molar conductivity decreases with decrease in concentration of electrolyte.
In the light of the above statements, choose the most appropriate answer from the options given below:
The variation of molar conductivity with concentration of an electrolyte (X) m aqueous solution is shown in the given figure.
The electrolyte X is ______.
Which of the following solutions of KCl will have the highest value of molar conductivity?
The following questions are case-based questions. Read the passage carefully and answer the questions that follow:
| Rahul set up an experiment to find the 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 the null point, he also calculated the conductivity K and molar conductivity ∧m and recorded his readings in tabular form. |
| S. No. | Conc. (M) |
k S cm−1 | ∧m S cm2 mol−1 |
| 1. | 1.00 | 111.3 × 10−3 | 111.3 |
| 2. | 0.10 | 12.9 × 10−3 | 129.0 |
| 3. | 0.01 | 1.41 × 10−3 | 141.0 |
Answer the following questions:
(a) Why does conductivity decrease with dilution? (1)
(b) If `∧_"m"^0` of KCl is 150.0 S cm2 mol−1, calculate the degree of dissociation of 0.01 M KCI. (1)
(c) If Rahul had used HCl instead of KCl then would you expect the ∧m values to be more or less than those per KCl for a given concentration? Justify. (2)
OR
(c) Amit a classmate of Rahul repeated the same experiment with CH3COOH solution instead of KCl solution. Give one point that would be similar and one that would be different in his observations as compared to Rahul. (2)
The solution of two electrolytes A and B are diluted. ^m of B increases 1.5 times while that of A increases 25 times. Which of the two is a strong electrolyte? Give a reason.
