Q18 Molar conductance of monobromoacetic acid at infinite dilution is calculated as x × 10-4 S m2 mol-1 at 25°C. The value of x (round off to the nearest integer)
Given:
| Electrolyte | Λ° (in 10-4 S m2 mol-1) |
|---|---|
| HBr | 429.5 |
| CH2BrCOOK | 113.4 |
| CH2BrCOOH | 117.2 |
Molar conductivity at infinite dilution for monobromoacetic acid (CH₃BrCOOH) is calculated using Kohlrausch’s law, which states that it equals the sum of ionic conductivities of H⁺ and CH₃BrCOO⁻. The value is found by combining given data: Λ°(HBr) – Λ°(KBr) + Λ°(CH₃BrKOOK). Rounding 390.03 to the nearest integer gives 390.
Problem Breakdown
Kohlrausch’s law applies here: Λ°(CH₃BrCOOH) = λ°(H⁺) + λ°(CH₃BrCOO⁻).
From HBr → λ°(H⁺) + λ°(Br⁻) = 427.95 S cm² mol⁻¹
From KBr → λ°(K⁺) + λ°(Br⁻) = 151.64 S cm² mol⁻¹, so λ°(H⁺) = 427.95 – 151.64 = 276.31 S cm² mol⁻¹
From CH₃BrKOOK → λ°(K⁺) + λ°(CH₃BrCOO⁻) = 113.72 S cm² mol⁻¹, so λ°(CH₃BrCOO⁻) = 113.72 – (151.64 – λ°(Br⁻)), but directly: Λ°(CH₃BrCOOH) = 427.95 – 151.64 + 113.72 = 390.03 ≈ 390.
Option Analysis
- (A) 164: Too low; ignores H⁺ contribution, possibly just acetate-related value. Incorrect.
- (B) 195: Underestimates; might subtract incorrectly without full ion sum. Incorrect.
- (C) 390: Matches exact calculation (427.95 – 151.64 + 113.72 = 390.03, nearest integer). Correct.
- (D) 467: Overestimates; perhaps adds without subtracting KBr properly. Incorrect.
Article Content
Molar conductance monobromoacetic acid infinite dilution is a key concept in electrochemistry for CSIR NET Life Sciences preparation. Using Kohlrausch law, calculate Λ°(CH₃BrCOOH) from given values: HBr (427.95), KBr (151.64), CH₃BrKOOK (113.72) S cm² mol⁻¹ at 25°C.
Kohlrausch Law Explained
Kohlrausch law states molar conductivity at infinite dilution equals sum of ionic conductivities, ideal for weak electrolytes like monobromoacetic acid.
Formula: Λ°(acid) = Λ°(HBr) – Λ°(KBr) + Λ°(salt), replacing common ions.
Step-by-Step Solution
- λ°(H⁺) = Λ°(HBr) – λ°(Br⁻) from Λ°(KBr) = λ°(K⁺) + λ°(Br⁻).
- λ°(CH₃BrCOO⁻) from Λ°(CH₃BrKOOK) – λ°(K⁺).
- Total: 427.95 – 151.64 + 113.72 = 390.03 ≈ 390 (nearest integer).
CSIR NET Exam Tips
- Practice similar problems with acetic acid analogs; options test common errors like incorrect subtraction.
- Strong electrolytes provide ionic data for weak acid calculations.
