Acid Base Equilibria

To form a buffer solution, we need:

• A weak acid and its conjugate base
• or a weak base and its conjugate acid

Now let's analyze each option: (a) 100 mL 0.1 M NaOH + 50 mL 0.1 M CH₃COOH
- CH₃COOH is a weak acid, NaOH is a strong base.
- Moles of CH₃COOH = mol
- Moles of NaOH = mol
- NaOH is in excess, completely neutralizes acid ⇒ no buffer

(b) 50 mL 0.1 M NaOH + 100 mL 0.1 M CH₃COOH
- CH₃COOH (weak acid), NaOH (strong base)
- Moles of CH₃COOH = mol
- Moles of NaOH = mol
- Partial neutralization ⇒ forms CH₃COONa ⇒ buffer

(c) 50 mL 0.1 M NH₄OH + 50 mL 0.1 M CH₃COOH
- Weak base (NH₄OH) and weak acid (CH₃COOH)
- Can act as a buffer system depending on strength balance ⇒ buffer

(d) 50 mL 0.1 M HCl + 100 mL 0.1 M CH₃COONa
- HCl is a strong acid, CH₃COONa is a conjugate base source
- Partial neutralization ⇒ forms CH₃COOH and CH₃COONa ⇒ buffer

Answer: (a) 100 mL 0.1 M NaOH + 50 mL 0.1 M CH₃COOH (no buffer formed)

Part (A): Comparing the Acid and Base Solutions

1. Substances: We have Tartaric Acid (an acid) and Washing Soda (Sodium Carbonate, Na₂CO₃, which forms a basic solution in water).
2. Concentration by Mass: Equal masses (1g) are dissolved in equal volumes (100 cc). So, the mass concentration (g/cc) is the same for both solutions.
3. Molar Mass:
   • Tartaric Acid (C₄H₆O₆): Molar mass ≈ 150 g/mol
   • Washing Soda (Na₂CO₃): Molar mass ≈ 106 g/mol
4. Molar Concentration: Since equal masses of substances with different molar masses are dissolved in equal volumes, their molar concentrations (moles/cc or Molarity) will be different. Specifically, the washing soda solution will have a higher molar concentration (1/106 > 1/150).
5. Strength: Tartaric acid is a weak acid. Sodium carbonate produces a basic solution through hydrolysis (CO₃²⁻ + H₂O ⇌ HCO₃⁻ + OH⁻) and acts as a base. The extent of dissociation (for the acid) and hydrolysis (for the base) determines the actual concentration of H⁺ and OH⁻ ions, respectively.
6. Acidity vs. Basicity: Because both the molar concentrations and the intrinsic strengths (Ka of tartaric acid vs. Kb of carbonate ion) are different, the resulting acidity (how low the pH) and basicity (how high the pH) will not be equal in magnitude compared to neutral pH 7. That is, |pH(acid) - 7| will likely not equal |pH(base) - 7|.
7. Conclusion (A): Taking one drop (equal small volume) of each solution means we are comparing solutions with different molar concentrations and different strengths. Therefore, the degree of acidity in the acid drop and the degree of basicity in the base drop will not be equal.

Correct Option for (A): 3. acidity in one and basicity in the other are not equal

Part (B): Mixing Phenolphthalein with Acidic Solution

1. Setup: We add phenolphthalein solution (prepared in basic medium, so it's initially pink) to the acidic tartaric acid solution.
2. Phenolphthalein Behavior: Phenolphthalein is an acid-base indicator. It is pink in basic solutions (approx. pH > 8.2) and colorless in acidic solutions (approx. pH < 8.2).
3. Mixing Process: When the pink, basic drops of phenolphthalein solution enter the bulk acidic solution:
   • The excess acid in the test tube neutralizes the small amount of base present in the indicator drops.
   • The phenolphthalein molecule, initially in its pink (basic) form, encounters an acidic environment.
   • In the acidic environment, phenolphthalein converts to its colorless form.
4. Observation: The pink color of the added indicator drops will disappear upon mixing with the tartaric acid solution because the overall solution remains acidic.

Correct Option for (B): 2. remains colourless as the colour of the added drops disappears