Question

An alkali is titrated against acid with methyl orange as an indicator, which of the following is a correct combination?

• A. Strong base is titrated by strong acid and end point is pinkish red to yellow.
• B. Weak base is titrated by strong acid and end point is yellow to pinkish red.
• C. Strong base is titrated by strong acid and end point is pink to colorless.
• D. Weak base is titrated by strong acid and end point is colorless to pink.

Answer 1

The correct option is B

Weak base is titrated by strong acid and end point is yellow to pinkish red.

Explanation for correct option:

B) Weak base is titrated by strong acid and end point is yellow to pinkish red.

• The pH range of methyl orange is $3.9$ to $4.5$.
• $\mathrm{pH}$ of weak bases is greater than $7$.
• When methyl orange is added to the weak base solution, the solution becomes yellow.
• When the solution is titrated by strong acid the pH becomes less than $3.1$and hence changes into pinkish red.

Explanation for incorrect option:

A) Strong base is titrated by strong acid and end point is pinkish red to yellow.

• Methyl orange gives yellow colour when added to the base, so the colour of the solution cannot change from pinkish red to yellow on titration.

C) Strong base is titrated by strong acid and end point is pink to colourless.

• Methyl orange gives yellow colour when added to the base, so the colour of the solution cannot change from pink to colourless on titration.

D) Weak base is titrated by strong acid and end point is colourless to pink.

• Methyl orange gives yellow colour when added to the base, so the colour of the solution cannot change from colourless to pink on titration.

Hence option B is correct, when a weak base is titrated with strong acid in presence of methyl orange the end point is when solution turns pinkish-red from yellow color.