Question

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

• A. Base $\to$ Weak , Acid $\to$ Strong , End point $\to$ Colourless to pink
• B. Base $\to$ Strong , Acid $\to$ Strong , End point $\to$ Pinkish red to yellow
• C. Base $\to$ Weak , Acid $\to$ Strong , End point $\to$ Yellow to pinkish red
• D. Base $\to$ Strong , Acid $\to$ Strong , End point $\to$ Pink to colourless

Answer 1

Answer: (C)

The $pH$ range of methyl orange is

$\underset{\text{Pinkish red}}{\leftarrow }3.9-4.5\underset{\text{Yellow}}{\leftarrow }$

Weak base is having $pH$ greater than $7.$ When methyl orange is added to weak base solution, the solution becomes yellow. This solution is titrated by strong acid and at the end point $pH$ will be less than $3.1$. Therefore solution becomes pinkish red.