The Question is from neet-chemistry organic-chemistry-basics purification-of-organic-compounds, and it was created by the author Mr. S M HUSSAIN

Question

A highly impure sample of benzoic acid is contaminated with naphthalene. Which of the following techniques would be LEAST effective for purifying the benzoic acid?

Mr. S M HUSSAIN · Accepted Answer

## CORE INFORMATION
✓ **Answer**: A
## CONCEPT FRAMEWORK
 **Primary Concept**: Purification techniques for organic compounds
 **Related Topics**: Recrystallization, distillation, sublimation, acid-base extraction
 **Prerequisites**: Understanding of solubility, boiling points, melting points, and acid-base chemistry.
## SOLUTION PATHWAY
**Given Information**:
  Benzoic acid is contaminated with naphthalene.
  We need to identify the least effective purification method.
**Method & Steps**:
 1. Analyze each purification technique's effectiveness in separating benzoic acid from naphthalene.
  Reasoning: Consider the properties of benzoic acid and naphthalene. Benzoic acid is a carboxylic acid with a relatively high melting point (122 °C) and moderate water solubility. Naphthalene is a nonpolar aromatic hydrocarbon with a lower melting point (80 °C) and very low water solubility.
 2. Evaluate each option:
  a) Simple distillation: This technique separates components based on their boiling points. Since both benzoic acid and naphthalene have relatively high boiling points, simple distillation may not be very effective in separating them, especially if they have similar boiling points or form an azeotrope. 
  b) Sublimation followed by recrystallization: Sublimation is effective for purifying solids that can directly transition from solid to gas.  Naphthalene sublimes easily, while benzoic acid sublimes less readily.  Recrystallization can further purify the benzoic acid after sublimation removes most of the naphthalene.
  c) Recrystallization from hot water: Benzoic acid has moderate solubility in hot water, while naphthalene has very low solubility.  Upon cooling, the benzoic acid will crystallize out leaving naphthalene behind. This is a relatively effective method.
  d) Treatment with sodium bicarbonate followed by acidification: Benzoic acid reacts with sodium bicarbonate to form a water-soluble sodium benzoate salt, while naphthalene remains insoluble.  Acidification then regenerates benzoic acid, leaving naphthalene behind in the solution. This is a very effective method.
 3. Identify the least effective method:
  Result: Simple distillation (option A) is the least effective method because it relies on boiling point differences, which may not be significant between benzoic acid and naphthalene.
  Verification: The other methods exploit differences in solubility and sublimation properties, providing better separation.
 ## OPTION ANALYSIS
A] ✓: Simple distillation is least effective because benzoic acid and naphthalene have relatively high and similar boiling points, making separation difficult.
B] :x: Sublimation separates naphthalene effectively, and recrystallization further purifies benzoic acid.
C] :x: Recrystallization from hot water exploits the difference in solubility between benzoic acid and naphthalene.
D] :x: Acid-base extraction effectively separates benzoic acid from naphthalene based on their different acid-base properties.
 ## LEARNING AIDS
 **Common Mistakes**:
  1.Assuming all purification techniques are equally effective: Different techniques are suitable for different compounds based on their properties.
  2.Overlooking the properties of the contaminants: The choice of purification method depends heavily on the nature of the contaminant.
 **Quick Tips**:
  1.Consider the boiling points, melting points, and solubility of both the desired compound and the contaminant.
  2.Acid-base extraction is very effective for separating acidic or basic compounds from neutral compounds.

Mr. S M HUSSAIN · Answer

The correct Option is "Simple distillation"

Mr. S M HUSSAIN · Answer

Sublimation followed by recrystallization

Mr. S M HUSSAIN · Answer

Recrystallization from hot water

Mr. S M HUSSAIN · Answer

Treatment with sodium bicarbonate followed by acidification

NEET Chemistry Organic Chemistry: Some basic principles and techniques Purification Of Organic Compounds MCQs

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