Question

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

Base	Acid	End point
Strong	Strong	Pink to colourless
Weak	Strong	Colourless to pink
Strong	Strong	Pinkish red to yellow
Weak	Strong	Yellow to pinkish red

• A. A
• B. B
• C. C
• D. D

Answer 1

Answer: (D)

Methyl orange is weak organic base. It is used in the titration of weak base vs strong acid.

$\underset{\underset{(\text{Yellow})}{\text{Unionized form }}}{\mathrm{MeOH}}\rightleftharpoons M{e}^{+}+\underset{\underset{(\text{Red})}{\text{Ionized form}}}{O{H}^{-}}$

In basic medium, equilibrium lies in backward direction and therefore it shows yellow colour.

In acidic medium, equilibrium shifts in forward direction and therefore, colour changes from yellow to red.