Question

$0.1N$ solution of $N{a}_{2}C{O}_3$ is being titrated with $0.1NHCl$, the best indicator to be used is:

• A. potassium ferricyanide
• B. phenolphthalein
• C. methyl orange
• D. litmus

Answer 1

The correct option is C methyl orange

Titration of ${Na}_2{CO}_3$ with $HCl$ involves following two reaction :

a) ${Na}_2{CO}_3+HCl\rightleftharpoons {NaHCO}_3+NaCl$

b) ${NaHCO}_3+HCl\rightleftharpoons NaCl+H_{2}O+{CO}_2\{H_2{CO}_3-carbonicacid\}$

We know phenolphthalein is an indicator and it works in the basic medium that is why it causes only $50$% of neutalisation of ${Na}_2{CO}_3$ because in the step $b$ the medium turns acidic due to formation of $H_2{CO}_3$ and phenolphatein will not work. On the other hand methyl orange is a basic indicator and works in the acidic medium and causes $100$% neutralisation of ${Na}_2{CO}_3$.

Answer will be $C$.