Question

Assertion: In titration of $N{a_2}C{O_3}$ with $HCl$ using methyl orange indicator, the volume required at the equivalence point is twice as that of acid required using phenolphthalein indicator.
Reason: Two moles of $HCl$ are required for complete neutralization of one mole of $N{a_2}C{O_3}$.
A.Both Assertion and reason are correct and reason is the correct explanation for assertion.
B.Both Assertion and reason correct but reason is not the correct explanation for assertion.
C.Assertion is correct but reason is incorrect.
D.Assertion is incorrect but reason is correct.

Answer 1

Titration (also known as titrimetry and volumetric analysis) is a common laboratory method of quantitative chemical analysis to determine the concentration of an identified analyte (a substance to be analyzed). A reagent, termed the titrant or titrator, is prepared as a standard solution of known concentration and volume. The titrant reacts with a solution of analyte (which may also be termed the titrand) to determine the analyte's concentration. The volume of titrant that reacted with the analyte is termed the titration volume.s

Complete Step By Step Solution:
In the titration of $N{a_2}C{O_3}$ with $HCl$ 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. This is because of the fact that phenolphthalein is a weak organic acid, and it changes its color in weakly basic medium $N{a_2}C{O_3}$ ​, so as soon as the $N{a_2}C{O_3}$ is converted to sodium bicarbonate phenolphthalein shows the color change indicating the completion of the reaction.
However, in the presence of methyl orange, all the basic substances left in the mixture will be neutralized by acid and methyl orange will show the color change when the medium is weakly acidic.
Methyl orange changes color in the pH range 3.0 to 4.4 when only one mole of $HCl$ reacts with one mole of sodium carbonate.
Phenolphthalein changes color in the pH range of 8.0 to 9.8 when one mole of sodium carbonate is completely neutralized with two moles of $HCl$.

Hence the correct option is (B).

Note: Methyl orange indicates at complete neutralisation while phenolphthalein indicates at first equivalence point. So, the volume of acid required is twice as much as methyl orange. An acid–base indicator (e.g., phenolphthalein) changes color depending on the p. Redox indicators are also used. A drop of indicator solution is added to the titration at the beginning; the endpoint has been reached when the color changes.