Vidaara.orgClass 12 · Chemistry
CodeVID-C12-03-T2-01
Conductance of Electrolytes — Assignment
Name: ____________________
Roll No.: __________
Date: ____________
General Instructions
- All questions are compulsory.
- Section A carries 1 mark each, Section B 2 marks, Section C 3 marks and Section D 5 marks.
- Show all steps in numericals and state units clearly. For full solutions, raise your doubts with your teacher.
Section A — Multiple Choice Questions
5 × 1 = 5 marks
1.
The reciprocal of resistivity is called:
- A.conductance
- B.conductivity
- C.cell constant
- D.molar conductivity
2.
The cell constant has units of:
- A.$\text{S cm}^{-1}$
- B.$\text{cm}^{-1}$
- C.$\text{S cm}^2\,\text{mol}^{-1}$
- D.$\Omega$
3.
Molar conductivity increases on dilution because:
- A.ions slow down
- B.more ions become available / move freely
- C.temperature drops
- D.conductivity rises
4.
Kohlrausch's law is strictly valid at:
- A.high concentration
- B.infinite dilution
- C.high temperature
- D.low temperature
5.
Which is a weak electrolyte?
- A.KCl
- B.NaCl
- C.CH3COOH
- D.HCl
Section B — Short Answer (2 marks)
3 × 2 = 6 marks
6.
Define conductivity and give its SI unit.
7.
Write the formula relating molar conductivity to conductivity.
8.
A $0.05\,\text{M}$ solution has $\kappa = 0.0124\,\text{S cm}^{-1}$. Find $\Lambda_m$.
Section C — Short Answer (3 marks)
2 × 3 = 6 marks
9.
State Kohlrausch's law and give two of its applications.
10.
Distinguish between the variation of $\Lambda_m$ with concentration for strong and weak electrolytes.
Section D — Long Answer (5 marks)
1 × 5 = 5 marks
11.
Define molar conductivity. Explain how it varies with concentration for strong and weak electrolytes and how Kohlrausch's law is used to obtain $\Lambda_m^0$ of a weak acid such as acetic acid.
Answer Key
Section A — Multiple Choice Questions
- (B) conductivity
- (B) $\text{cm}^{-1}$
- (B) more ions become available / move freely
- (B) infinite dilution
- (C) CH3COOH
Section B — Short Answer (2 marks)
- Conductance of a solution between electrodes of unit area unit distance apart; unit $\text{S m}^{-1}$ (or $\text{S cm}^{-1}$).
- $\Lambda_m = \frac{\kappa \times 1000}{C}$, with $\kappa$ in $\text{S cm}^{-1}$ and $C$ in $\text{mol L}^{-1}$.
- $\Lambda_m = \frac{0.0124 \times 1000}{0.05} = 248\,\text{S cm}^2\,\text{mol}^{-1}$.
Section C — Short Answer (3 marks)
- $\Lambda_m^0 = \nu_+\lambda_+^0 + \nu_-\lambda_-^0$; used to find $\Lambda_m^0$ of weak electrolytes and degree of dissociation $\alpha = \Lambda_m/\Lambda_m^0$.
- Strong: small linear rise with $\sqrt{C}$, extrapolates to $\Lambda_m^0$. Weak: steep rise near zero C, no direct extrapolation.
Section D — Long Answer (5 marks)
- $\Lambda_m = \frac{\kappa \times 1000}{C}$; strong electrolytes give $\Lambda_m = \Lambda_m^0 - A\sqrt{C}$, weak ones rise steeply; $\Lambda_m^0(\text{CH}_3\text{COOH}) = \Lambda_m^0(\text{CH}_3\text{COONa}) + \Lambda_m^0(\text{HCl}) - \Lambda_m^0(\text{NaCl})$.
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