Question:

Incorrect statement for the use of indicators in acid-base titration is :

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Always choose the indicator based on the pH range at the equivalence point of the titration.
Updated On: Mar 21, 2025
  • Methyl orange may be used for a weak acid vs weak base titration.
  • Phenolphthalein may be used for a strong acid vs strong base titration.

  • Methyl orange is a suitable indicator for a strong acid vs weak base titration.
  • Phenolphthalein is a suitable indicator for a weak acid vs strong base titration.
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The Correct Option is A

Approach Solution - 1

IndicatorpH range
Methyl orange3.2 - 4.5 
Phenolpthalein8.3 - 10.5 

Methyl orange may be used for a strong acid vs strong base and strong acid vs weak base titration. Phenolpthalein may be used for a strong acid vs strong base and weak acid vs strong base titration
So , the correct option is (A) : Methyl orange may be used for a weak acid vs weak base titration.

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Approach Solution -2

The suitable indicators for different types of acid-base titrations are:

  • Methyl orange is generally used for titrations involving strong acid vs weak base because it changes color in the acidic pH range (red at pH < 3.4 and yellow at pH > 4.4).
  • Phenolphthalein is suitable for titrations involving weak acids vs strong bases as it changes color in the basic pH range (colorless below pH 4.8 and pink above pH 6.4).
  • The statement that methyl orange may be used for weak acid vs weak base titration is incorrect because it is not suitable for that combination. For weak acid vs weak base titrations, neutral red or bromothymol blue would be more appropriate.

Thus, the incorrect statement is option (1).

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Questions Asked in JEE Main exam

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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 PropertiesACIDSBASES
TasteSourBitter
Colour on Litmus paper Turns blue litmus redTurns red litmus blue
Ions produced on dissociationH+OH-
pH<7 (less than 7)>7 (more than 7)
Strong acidsHCl, HNO3, H2SO4NaOH, KOH
Weak AcidsCH3COOH, H3PO4, H2CO3NH4OH

Chemical Properties of Acids and Bases

Type of ReactionAcidBases
Reaction with Metals

Acid + Metal → Salt + Hydrogen gas (H2)

E.g.,

Zn(s)+ dil. H2SO4 → ZnSO(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.

Read more on Acids, Bases and Salts