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Ionic Equilibrium question

2018 · Shift 0 · Q2
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Ionic Equilibrium question

2018 · Shift 0 · Q2

JEE MainChemistryIonic EquilibriumMCQ+4 / −1
An alkali is titrated against an acid with methyl orange as indicator, which of the following is a correct combination?
  1. A
    Base Acid End point
    Weak Strong Colourless to pink
  2. B
    Base Acid End point
    Strong Strong Pinkish red to yellow
  3. C
    Base Acid End point
    Weak Strong Yellow to pinkish red
  4. D
    Base Acid End point
    Strong Strong Pink to colourless
View written solutionFree

Correct answer: C

  1. Recall the colour change of methyl orange

    Methyl orange behaves as:

    • Red/pinkish red in acidic medium
    • Yellow in alkaline medium

    Therefore, when an alkali (base) is titrated against an acid, the solution initially contains base, so the indicator colour is yellow. As acid is added, the medium becomes acidic near the end point, so the colour changes to pinkish red.

  2. Nature of titration suitable for methyl orange

    Methyl orange is suitable for titrations where the end point lies in the acidic range, especially:

    • Strong acid vs weak base
    • also usable in strong acid vs strong base

    Since the question says an alkali is titrated against an acid, acid is added to base. For methyl orange to be the proper indicator here, the base should be weak and the acid should be strong.

  3. Check each option

    A: Weak base, strong acid, but colourless to pink

    • Wrong colour change. Methyl orange is not colourless/pink.

    B: Strong base, strong acid, pinkish red to yellow

    • Wrong direction of colour change. Since initial solution is basic, it should go yellow to pinkish red, not the reverse.

    C: Weak base, strong acid, yellow to pinkish red

    • Correct base-acid combination and correct colour change.

    D: Strong base, strong acid, pink to colourless

    • Wrong colour behaviour for methyl orange.

  4. Final answer

    The correct combination is Option C.

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