CHM-DSC-253: Section C (Physical Practical)
Paper Name: Practical (Inorganic, Organic and Physical Chemistry)
Section Marks: 20 Marks
1. Solubility of Benzoic Acid & Enthalpy Calculation
This experiment involves determining how the solubility of benzoic acid changes with temperature to calculate thermodynamic parameters.
- Method: Saturated solutions of benzoic acid are prepared at different temperatures and titrated against standard NaOH.
- Calculation: By plotting log S (solubility) against 1/T (reciprocal of temperature), the enthalpy of dissolution (ΔH) can be determined using the Van't Hoff equation.
2. Preparation of Buffer Solutions
Buffers are essential for maintaining constant pH in chemical and biological systems.
- Acidic Buffer: Preparation of Sodium acetate-acetic acid buffer mixtures of different pH values.
- Basic Buffer: Preparation of Ammonium chloride-ammonium hydroxide buffer solutions.
- Measurement: The pH of prepared buffers is verified using a pH meter or universal indicator.
3. pH-metric Titrations
Using a pH meter provides a more precise end point than visual indicators for acid-base reactions.
- Strong Acid vs Strong Base: Titration of HCl against NaOH.
- Data Analysis: The pH is recorded after each addition of base. A plot of pH vs. Volume of base added shows a sharp vertical rise at the equivalence point.
- First Derivative: Plotting ΔpH/ΔV vs. V (average volume) helps in precisely locating the inflection point.
4. Critical Solution Temperature (CST)
CST is the temperature at which two partially miscible liquids become completely miscible in all proportions.
- System: Phenol-Water system.
- Procedure: Mixtures of phenol and water in different ratios are heated until they become clear and then cooled until they become turbid.
- Result: The maximum temperature on the temperature-composition curve is the CST (approx. 66°C for phenol-water).
5. Exam Focus: Lab Performance & Viva
Practical Performance Tips
- pH Meter Calibration: Always calibrate the pH meter using standard buffer tablets (pH 4, 7, and 9.2) before starting your titration.
- Temperature Control: For CST and solubility experiments, ensure slow and uniform heating/cooling to capture accurate temperature readings.
- Graphing: Ensure your graphs have labeled axes, units, and a clear title. Use a sharp pencil for plotting points.
Common Viva Questions
Q: What is the Van't Hoff equation for solubility?
A: It relates the change in equilibrium constant (or solubility) to the change in temperature.
Q: Why do we use a salt bridge in some potentiometric measurements?
A: To eliminate liquid junction potential and complete the circuit without mixing the solutions.
Q: Define Critical Solution Temperature.
A: The temperature at which two partially miscible liquids become completely miscible.
Q: What is a buffer?
A: A solution that resists changes in pH upon the addition of small amounts of acid or base.