Question

A 20 ml solution of 0.2 N $N{a}_{2}C{O}_3$ is titrated with 0.1 N $HCl$ with phenolphthalein as the indicator. Another 20 ml solution of 0.2 N $N{a}_{2}C{O}_3$ is titrated with 0.1 N $HCl$ with methyl orange as the indicator. The volume of $HCl$ required for both the titrations resepectively is:

• A. 20 mL and 40 mL
• B. 40 mL and 20 mL
• C. 40 mL and 40 mL
• D. 20 mL and 20 mL

Answer 1

The correct option is A 20 mL and 40 mL

Phenolphthalein indicator changes colour at the half equivalence point i.e., conversion of sodium carbonate to sodium bicarbonate.
However methyl orange changes colour at the complete neutralisation point.
Volume of sodium carbonate = 20 mL
Normality of sodium carbonate = 0.2 N
Normality of $HCl$ = 0.1 N
Volume of $HCl$ $=\frac{N_{N{a}_{2}C{O}_3}\times V_{N{a}_{2}C{O}_3}}{N_{HCl}}$
For complete neutralisation:
Volume of $HCl$ $=\frac{20\times 0.2}{0.1}=40\text{mL}$
For the half equivalence point volume $=20\text{mL}$