Question

Assertion: In the titration of \[{\rm{N}}{{\rm{a}}_{\rm{2}}}{\rm{C}}{{\rm{O}}_{\rm{3}}}\] with \[{\rm{HCl}}\] , using methyl orange indicator, the volume of the acid required at the equivalence point is twice that of the acid required using phenolphthalein as indicator.
Reason: Two moles of \[{\rm{HCl}}\] are required for the complete neutralization of one mole of \[{\rm{N}}{{\rm{a}}_{\rm{2}}}{\rm{C}}{{\rm{O}}_{\rm{3}}}\].
A ) Both assertion and reason are correct and reason is the correct explanation for the assertion.
B ) Both assertion and reason are correct but reason is not the correct explanation for assertion.
C ) Assertion is correct but reason is incorrect.
D ) Both assertion and reason are incorrect.

Answer 1

Hint: Write a balanced chemical equation for the titration reaction and consider the acid base properties of phenolphthalein and methyl orange indicators. In the titration, there are two endpoints. Out of the two indicators, one will detect only one endpoint. Whereas the other indicator will detect both end points.

Complete answer:
Write the balanced chemical equations for the titration of sodium carbonate with hydrochloric acid.

${\rm{N}}{{\rm{a}}_{\rm{2}}}{\rm{C}}{{\rm{O}}_{\rm{3}}}{\rm{ + HCl }} \to {\rm{ NaHC}}{{\rm{O}}_3}{\rm{ + NaCl }}$
${\rm{NaHC}}{{\rm{O}}_3}{\rm{ + HCl }} \to {\rm{ }}{{\rm{H}}_2}{\rm{C}}{{\rm{O}}_3}{\rm{ + NaCl }}$
${\rm{\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_}}$
${\rm{N}}{{\rm{a}}_{\rm{2}}}{\rm{C}}{{\rm{O}}_{\rm{3}}}{\rm{ + 2 HCl }} \to {\rm{ }}{{\rm{H}}_2}{\rm{C}}{{\rm{O}}_3}{\rm{ + 2 NaCl}}$

> One mole of sodium carbonate reacts with one mole of hydrochloric acid to form one mole of sodium hydrogen carbonate and one mole of sodium chloride. One mole of sodium hydrogen carbonate reacts with one mole of hydrochloric acid to form one mole of carbonic acid. In the net reaction, one mole of sodium carbonate reacts with two moles of hydrochloric acid to form one mole of carbonic acid and two moles of sodium chloride. Hence, the reason is correct. Carbonic acid formed in the reaction can further break into carbon dioxide and water.
> Thus, there are two end points in the titration of sodium carbonate with hydrochloric acid. The first end point is the formation of sodium hydrogen carbonate. The second end point is the formation of carbonic acid.
> When a phenolphthalein indicator is used, only the first equivalence point is detected. This is because phenolphthalein is a weak organic acid. It changes colour in a weakly basic medium. When sodium carbonate is converted to sodium hydrogen carbonate, the medium is weakly basic. Hence, when the first end point is obtained, the phenolphthalein indicator shows colour change.
> When a methyl orange indicator is used, complete conversion of sodium carbonate to carbonic acid is obtained. Methyl orange shows colour change when the medium becomes weakly acidic. At the second end point, the medium is weakly acidic.
> Thus, in presence of phenolphthalein indicator, one mole of sodium carbonate reacts with one mole of hydrochloric acid whereas in presence of methyl orange indicator, one mole of sodium carbonate reacts with two moles of hydrochloric acid.
Hence, the assertion is also correct.

Hence, the correct option is the option B ) Both assertion and reason are correct but reason is not correct explanation for assertion.

Note: Suppose a mixture containing sodium carbonate and sodium hydrogen carbonate is present. Then using the titration of this mixture with hydrochloric acid in presence of phenolphthalein and methyl orange indicators, it is possible to determine the percentage of sodium carbonate and sodium hydrogen carbonate in the mixture.