JEE Main 2021 · 26 August · morning · Single correct · +4 / −1
Given below are two statements :
Statement 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 :
- (a) Statement I is true but Statement II is false.
- (b) Statement I is false but Statement II is true.
- (c) Both Statement I and Statement II are true.
- (d) Both Statement I and Statement II are false.
Answer and solution
Answer (d) Both Statement I and Statement II are false.
Solution
Chemistry ma’am · 4 steps
At infinite dilution both are fully ionised, so only ion speed matters, and H⁺ is the fastest ion.
- Statement I. Limiting molar conductivity Λₘ° is measured at infinite dilution, where even a weak electrolyte is fully dissociated. So strong or weak no longer matters. Only how fast the ions move does, and H⁺ moves much faster than K⁺.
Electrolyte Ions Λₘ° (S cm² mol⁻¹) KCl 73.5 + 76.3 about 150 CH₃COOH 349.6 + 40.9 about 390 - So I is false. CH₃COOH has the higher limiting molar conductivity, not KCl.
- Statement II. Molar conductivity increases as concentration decreases. A strong electrolyte has less interionic attraction on dilution, and a weak electrolyte dissociates more. So Statement II is false.
- So. Both statements are false, which is option (d).
The idea to keep. Strong or weak affects Λₘ at real concentrations, not Λₘ°. At infinite dilution, compare ion mobilities. And molar conductivity always rises on dilution.
Answer from the NTA final answer key. Solution by PrepWiser.