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

$\xleftarrow[\text{Pinkish red}]{} 3.9 - 4.5 \xleftarrow[\text{Yellow}]{}$

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.