Question

When $N{a_2}C{O_3}$ solution is titrated against HCl solution , the indicator used is :
a) phenolphthalein
b) methyl orange
c) methyl red
d) starch

Answer 1

Hint: The decomposition of $N{a_2}C{O_3}$ by acid HCl takes place in 2 steps. In the first step, solution is basic and in the final step, carbonic acid is produced, so solution is acidic .so we need that indicator which changes its colour in both acidic as well as basic range medium.

Complete step by step answer:
Indicators are weak organic acids or bases. They show different colours at different pH ranges. So, they are used during titrations to detect the neutralisation process. They change their colour in different mediums which helps the chemist to know the result of titrations.
For example, phenolphthalein is colourless in acidic medium while shows red/pink colour in basic medium. Its indicator range is 8.3 to 10.0. Thus, phenolphthalein works only in the basic medium.
Methyl orange is red in acidic medium while yellow colour in basic medium. Its pH indicator range is 3.1 to 4.4. Thus, methyl orange can work in both basic as well as acidic medium because it can show colour change in both mediums.

Titration of $N{a_2}C{O_3}$ with HCl takes place in two steps:
\[\begin{gathered}
  a) N{a_2}C{O_3} + HCl \rightleftharpoons NaHC{O_3} + NaCl
  b) NaHC{O_3} + HCl \rightleftharpoons NaCl + {H_2}O + C{O_2}
\end{gathered} \]
$C{O_2}$ is from ${H_2}C{O_3}$ (carbonic acid)

In the first step, medium is basic and pH will be near around 8 or 9 while in step second medium of solution is acidic. After the complete neutralisation in the second step, the next drop of HCl will bring down the pH to near about 3 or 4. So we need that indicator which can change colour in this range.
And we know methyl orange shows colour change at pH range 3.1 to 4.4, thus it will be the suitable indicator for this titration.
So, the correct answer is “Option B”.

Note: Indicator shows colour change in a single excess drop after neutralisation is done. Thus indicators help us to determine the end point of the titrations. In the titration of $N{a_2}C{O_3}$ with HCl, pH change is relatively slow near the neutralisation point , therefore the colour change is not much sharp and the titration needs to be done slowly.