Question

Your teacher uses a weighing balance to take equal amounts of two substances, tartaric acid and washing soda, say 1g. Each is dissolved separately into 100 cc of water.

(A) In 1 drop of the acid solution and 1 drop of the basic solution, we have:
1. equal amount of acid and base respectively
2. equally acidic and basic substance respectively
3. acidity in one and basicity in the other are not equal
4. equal magnitude of the quantity —pH-7—
(B) Take a few cc of the acidic solution in a test-tube and mix a few drops of coloured
phenolphthaline solution (prepared in basic medium) into it. Which of the following may be 4
happening:
1. The colour of the solution instantly changes pink
2. remains colourless as the colour of the added drops disappears
3. the colour diffuses through the solution and finally disappears
4. the colour diffuses through the solution and finally the entire solution acquires a faint pink
colour.

• (A) 3, (B) 2
• (A) 3, (B) 3

Hint:

Phenolphthalein is a pH indicator that is colourless in acidic solutions and turns pink in basic solutions (pH>8.4).

Answer 1

The Correct Option is A

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

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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

Answer 2

(A) Even though 1 gram each of tartaric acid and washing soda are dissolved in 100 cc water, their acidic and basic strengths are different due to their different molar masses and dissociation behavior. Hence: 
Correct answer: 3. acidity in one and basicity in the other are not equal

(B) Phenolphthalein is colorless in acidic medium and turns pink in basic medium. Since a few drops of phenolphthalein solution prepared in a basic medium are added to an acidic solution, the basic color fades as it reacts with acid: 
Correct answer: 2. remains colourless as the colour of the added drops disappears