CHM-DSC-253: Section A (Inorganic Practical)

Paper Name: Practical (Inorganic, Organic and Physical Chemistry)

Contact Hours: 60; Credits: 04

1. Principles of Volumetric Titrations

Volumetric analysis involves determining the volume of a solution of known concentration (titrant) required to react completely with a measured volume of a solution of unknown concentration (analyte).

Key Components

• Standard Solution: A solution whose concentration is exactly known.
• Equivalence Point: The point at which the added titrant is chemically equivalent to the analyte.
• End Point: The physical change (usually color change) that indicates the titration is complete.

2. Titrations with Potassium Permanganate (KMnO4)

Potassium permanganate is a powerful oxidizing agent used in acidic media (usually H2SO4).

Determination of Oxalic Acid

• Reaction: 2 MnO4⁻ + 5 C2O4²⁻ + 16 H⁺ → 2 Mn²⁺ + 10 CO2 + 8 H2O.
• Procedure Note: The solution must be heated to about 60-70°C before titration to speed up the reaction.
• Indicator: KMnO4 acts as a self-indicator; the end point is the appearance of a permanent pale pink color.

Determination of Iron (II)

• Reaction: MnO4⁻ + 5 Fe²⁺ + 8 H⁺ → Mn²⁺ + 5 Fe³⁺ + 4 H2O.
• Application: Used to determine the strength of ferrous ammonium sulphate or other Fe(II) salts.

3. Titrations with Potassium Dichromate (K2Cr2O7)

Unlike KMnO4, K2Cr2O7 is a primary standard and is very stable in solution.

• Indicator: It is not a self-indicator. Internal indicators like N-phenylanthranilic acid or Diphenylamine are used.
• End Point: A sharp color change from green to violet-red (with N-phenylanthranilic acid).

4. Determination of Alkali in Soap/Detergents

This practical involves measuring the total free alkali (NaOH or KOH) present in commercial soap samples.

• Method: Acid-base back titration or direct titration of the soap solution with a standard acid (like HCl).
• Significance: Excess alkali can be harmful to skin/fabrics, so its percentage must be within standard limits.

5. Water of Crystallization in Mohr's Salt

Mohr's salt is Ferrous Ammonium Sulphate, FeSO4.(NH4)2SO4.xH2O.

• Goal: To determine the value of 'x' (the number of water molecules).
• Method: Titrating a known mass of the salt against a standardized KMnO4 solution.
• Calculation: Based on the molar ratio of Fe²⁺ to MnO4⁻.

6. Lab Exam Focus & Viva Questions

Practical Exam Tips

• Reading the Burette: For colored solutions like KMnO4, read the upper meniscus.
• Heating: Do not boil the oxalic acid solution; just heat it until steam starts to rise (approx. 70°C).
• Internal Assessment: Remember that 30 marks are for internal assessment, which includes your lab notebook regularity (5 marks) and attendance (10 marks).

Common Viva Questions

Q: Why is H2SO4 used instead of HCl in permanganate titrations?
A: HCl is itself an reducing agent and would react with KMnO4, leading to incorrect results.

Q: What is a primary standard?
A: A substance that is available in high purity, stable, and can be used to prepare a solution of known concentration directly by weighing (e.g., Oxalic acid, K2Cr2O7).

Q: Why does the first drop of KMnO4 take time to decolorize?
A: The reaction is slow initially; however, the Mn²⁺ ions formed act as an autocatalyst, speeding up the subsequent reaction.