Sample 1Medium
A graph of molar conductivity Λm (y-axis) against √c (x-axis) for two electrolytes in water shows line P, nearly straight with a small negative slope, meeting the y-axis at a definite value, and curve Q, low and flat at higher √c but rising very steeply as √c approaches zero, so it cannot be traced to the axis. Which statement is correct?
- A
Q is acetic acid, and its limiting molar conductivity is obtained from Kohlrausch's law
- B
P is acetic acid, and its limiting molar conductivity is read from the intercept
- C
Q is potassium chloride, a strong electrolyte
- D
Q is acetic acid, and its limiting molar conductivity is found by extending curve Q to the y-axis
Show the answer
The answer is A.A strong electrolyte such as KCl gives a nearly straight line whose intercept is Λ°m. A weak electrolyte such as acetic acid dissociates much more on dilution, so its curve rises steeply and Λ°m is found from Kohlrausch's law of independent migration of ions.
- B.
A nearly linear plot with a definite intercept belongs to a strong electrolyte such as KCl.
- C.
A strong electrolyte gives the nearly straight line P; the steep curve Q is typical of a weak electrolyte.
- D.
Because Q rises so steeply near zero concentration, it cannot be extended to the axis; Λ°m must come from Kohlrausch's law instead.
NCERT: Class 12 Chemistry, Chapter 2
Sample 2NEET level
At infinite dilution, the limiting molar conductivities of HCl, NaCl and CH3COONa are 425.9, 126.4 and 91.0 S cm² mol⁻¹ respectively. The limiting molar conductivity of acetic acid is
- A
390.5 S cm² mol⁻¹
- B
643.3 S cm² mol⁻¹
- C
461.3 S cm² mol⁻¹
- D
217.4 S cm² mol⁻¹
Show the answer
The answer is A.By Kohlrausch's law, Λ°m(CH3COOH) = Λ°m(HCl) + Λ°m(CH3COONa) − Λ°m(NaCl). Λ°m(CH3COOH) = 425.9 + 91.0 − 126.4 = 390.5 S cm² mol⁻¹.
- B.
Adds all three values instead of subtracting NaCl.
- C.
Subtracts CH3COONa and adds NaCl, the reverse combination.
- D.
Adds only NaCl and CH3COONa, ignoring HCl.
NCERT: Class 12 Chemistry, Unit 2 (Electrochemistry), Conductance of Electrolytic Solutions: Applications of Kohlrausch law