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Question
Suggest a way to determine the `∧_"m"^∘`value of water.
Solution
Water is a weak electrolyte. Its `∧_"m"^∘`value can be determined with the help of Kohlrausch's law.
`∧_"m"^∘("HCl") = ∧_"m"^∘("H"^+) + ∧_"m"^∘("Cl"^-)` ...(i)
`∧_"m"^∘("NaOH") = ∧_"m"^∘("Na"^+) + ∧_"m"^∘("OH"^-)` ...(ii)
`∧_"m"^∘("NaCl") = ∧_"m"^∘("Na"^+) + ∧_"m"^∘("Cl"^-)` ...(iii)
By adding (i) and (ii) and subtracting (iii) we get
`∧_"m"^∘("H"_2"O") = ∧_"m"^∘("H"^+) + ∧_"m"^∘("OH"^-)`
= `∧_"m"^∘("HCl") + ∧_"m"^∘("NaOH") - ∧_"m"^∘("NaCl")`
RELATED QUESTIONS
State Kohlrausch’s law of independent migration of ions.
State Kohlrausch law of independent migration of ions.
The conductivity of sodium chloride at 298 K has been determined at different concentrations and the results are given below:
| Concentration/M | 0.001 | 0.010 | 0.020 | 0.050 | 0.100 |
| 102 × κ/S m−1 | 1.237 | 11.85 | 23.15 | 55.53 | 106.74 |
Calculate `∧_"m"`for all concentrations and draw a plot between `∧_"m"`and `"c"^(1/2)`. Find the value of `∧_"m"^0`.
How can you determine limiting molar conductivity, 0 m for strong electrolyte and weak electrolyte?
A steady current of 2 amperes was passed through two electrolytic cells X and Y connected in series containing electrolytes FeSO4and ZnSO4 until 2.8g of Fe deposited at the cathode of cell X. How long did the current flow? Calculate the mass of Zn deposited at the cathode of cell Y.
(Molar mass: Fe=56g mol-1,Zn=65.3g mol-1,1F=96500C mol-1)
\[\ce{Λ^0_m H2O}\] is equal to:
(i) \[\ce{Λ^0_m_{(HCl)} + \ce{Λ^0_m_{(NaOH)} - \ce{Λ^0_m_{(NaCl)}}}}\]
(ii) \[\ce{Λ^0_m_{(HNO_3)} + \ce{Λ^0_m_{(NaNO_3)} - \ce{Λ^0_m_{(NaOH)}}}}\]
(iii) \[\ce{Λ^0_{(HNO_3)} + \ce{Λ^0_m_{(NaOH)} - \ce{Λ^0_m_{(NaNO_3)}}}}\]
(iv) \[\ce{Λ^0_m_{(NH_4OH)} + \ce{Λ^0_m_{(HCl)} - \ce{Λ^0_m_{(NH_4Cl)}}}}\]
Assertion: Copper sulphate can be stored in zinc vessel.
Reason: Zinc is less reactive than copper.
The molar conductivity of 0.007 M acetic acid is 20 S cm2 mol−1. What is the dissociation constant of acetic acid? Choose the correct option.
`[(Λ_("H"^+)^ο = 350 "S" "cm"^2 "mol"^-1), (Λ_("CH"_3"COO"^-)^ο = 50 "S" "cm"^2 "mol"^-1)]`
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Reason (R): When concentration approaches zero, the molar conductivity is known as limiting 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)
