A compound ‘X’ is a weak acid and it exhibits colour change at pH close to the equivalence point during neutralization of NaOH with CH3COOH. Compound ‘X’ exists in ionized from in basic medium. The compound ‘X’ is
- Methyl orange
- Methyl red
- Phenolphthalein
- Eriochrome Black T
The Correct Option is C
Approach Solution - 1
The question involves determining which compound acts as an indicator that exhibits a color change at the pH close to the equivalence point during the neutralization of NaOH (a strong base) with CH3COOH (a weak acid), and exists in ionized form in basic medium.
- Identification of equivalence point pH:
- During the neutralization of a strong base like NaOH with a weak acid like CH3COOH, the equivalence point will occur at a pH greater than 7 because the acetate ion (CH3COO-) acts as a weak base in water.
- Properties of given indicators:
- Methyl orange: Changes color at pH 3.1 to 4.4.
- Methyl red: Changes color at pH 4.4 to 6.2.
- Phenolphthalein: Changes color at pH 8.2 to 10.0.
- Eriochrome Black T: Commonly used in complexometric titrations, does not change at the right pH range here.
- Analysis and Conclusion:
- Since the equivalence point for the titration of CH3COOH with NaOH is above 7, Phenolphthalein is the suitable indicator.
- Phenolphthalein changes color around the basic pH where it is ionized, transitioning from colorless in acidic form to pink in its basic form. This suits the requirement of color change at the equivalence point and its existence in ionized form in basic medium.
Therefore, the correct answer is Phenolphthalein.
Approach Solution -2
In basic medium,
[H⨁] decreases and therefore more of (Ph⊖) is produced.
So, the correct option is (C): Phenolphthalein.
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Concepts Used:
Acids and Bases
Acid is any hydrogen-containing substance that is capable of donating a proton (hydrogen ion) to another substance. Base is an ion or molecule capable of accepting a hydrogen ion from acid.
Physical Properties of Acids and Bases
| Physical Properties | ACIDS | BASES |
| Taste | Sour | Bitter |
| Colour on Litmus paper | Turns blue litmus red | Turns red litmus blue |
| Ions produced on dissociation | H+ | OH- |
| pH | <7 (less than 7) | >7 (more than 7) |
| Strong acids | HCl, HNO3, H2SO4 | NaOH, KOH |
| Weak Acids | CH3COOH, H3PO4, H2CO3 | NH4OH |
Chemical Properties of Acids and Bases
| Type of Reaction | Acid | Bases |
| Reaction with Metals | Acid + Metal → Salt + Hydrogen gas (H2) E.g., Zn(s)+ dil. H2SO4 → ZnSO4 (Zinc Sulphate) + H2 | Base + Metal → Salt + Hydrogen gas (H2) E.g., 2NaOH +Zn → Na2ZnO2 (Sodium zincate) + H2 |
| Reaction with hydrogen carbonates (bicarbonate) and carbonates | Metal carbonate/Metal hydrogen carbonate + Acid → Salt + Carbon dioxide + Water E.g., HCl+NaOH → NaCl+ H2O 2. Na2CO3+ 2 HCl(aq) →2NaCl(aq)+ H2O(l) + CO2(g) 3. Na2CO3+ 2H2SO4(aq) →2Na2SO4(aq)+ H2O(l) + CO2(g) 4. NaHCO3+ HCl → NaCl+ H2O+ CO2 | Base+ Carbonate/ bicarbonate → No reaction |
| Neutralisation Reaction | Base + Acid → Salt + Water E.g., NaOH(aq) + HCl(aq) → NaCl(aq) + H2O(l) | Base + Acid → Salt + Water E.g., CaO+ HCl (l) → CaCl2 (aq)+ H2O (l) |
| Reaction with Oxides | Metal oxide + Acid → Salt + Water E.g., CaO+ HCl (l) → CaCl2 (aq)+ H2O (l) | Non- Metallic oxide + Base → Salt + Water E.g., Ca(OH)2+ CO2 → CaCO3+ H2O |
| Dissolution in Water | Acid gives H+ ions in water. E.g., HCl → H+ + Cl- HCl + H2O → H3O+ + Cl– | Base gives OH- ions in water. |
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