Question

A 20 ml solution of 0.1 N $N{a}_{2}C{O}_3$ is titrated with 0.1N HCl with phenolphthalein as the indicator. Another 20 ml solution of 0.1 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 20 mL
• B. 10 mL and 20 mL
• C. 10 mL and 10 mL
• D. 20 mL and 10 mL

Answer 1

The correct option is D 20 mL and 10 mL

Methyl orange indicator changes color at the half equivalence point i.e conversion of sodium carbonate to sodium bicarbonate.
However phenolphthalein changes color at the complete neutralisation point.
Volume of sodium carbonate = 20 ml
Normality of sodium carbonate = 0.1 N
Normality of HCl = 0.1 N
Volume of HCl $=\frac{N_{NaOH}\times V_{NaOH}}{N_{HCl}}$
For complete neutralisation:
Volume of HCl = $\frac{20\times 0.1}{0.1}=20mL$
For the half equivalence point volume = 10 mL