Question

During the titration of mixture of $NaOH$, $N{{a}_{2}}C{{O}_{3}}$ and an inert substance against hydrochloric acid:
(A)Phenolphthalein is used to detect end point when $NaOH$ is completely neutralized and half of is neutralized
(B)Methyl orange is used to detect the final end point
(C) Methyl orange is used to detect the first end point
(D) Phenolphthalein is used to detect the final end point.

Answer 1

Strong acid reacts with weak base; this will result in an acidic solution so $pH$ will be less than 7. In this titration, Indicator methyl orange that changes its basic color to acidic color is a good choice. Phenolphthalein is used as an indicator for Strong acid -strong base titration. Reaction of sodium hydroxide and hydrochloric occurs in one step. Reaction between sodium carbonate and hydrochloric acid occurs in two steps.

Complete solution step by step:
Equivalence point is a point in chemical reaction when chemically equivalent quantities of acid and base are mixed.
In mixture of Sodium Hydroxide and Sodium Carbonate,
Reaction between hydrochloric acid and sodium carbonate takes place in two stages

& HCl+N{{a}_{2}}C{{o}_{3}}\to NaHC{{O}_{3}}+{{H}_{2}}O \\\ & HCl+NaHC{{O}_{3}}\to NaCl+C{{O}_{2}}+{{H}_{2}}O \\\ \end{aligned}$$ The solution mixture of stage 1 at equivalence point is alkaline which can be detected by indicator phenolphthalein. The solution mixture of stage 2 at the equivalence point is acidic which can be detected by indicator methyl orange. Reaction of Hydrochloric acid takes place in one step. $HCl+NaOH\to NaCl+{{H}_{2}}O$ Solution mixture of this reaction at the equivalence point is neutral which can be indicated by phenolphthalein. When sodium hydroxide is completely neutralized, half of sodium carbonate will be half neutralized. it will be indicated by phenolphthalein, When sodium carbonate will be completely neutralized, it will be indicated by methyl orange. During the titration of mixture of$NaOH$, $N{{a}_{2}}C{{O}_{3}}$ and an inert substance against hydrochloric acid (A) Phenolphthalein is used to detect end point when $NaOH$is completely neutralized and half of $N{{a}_{2}}C{{O}_{3}}$ is neutralized and (B)Methyl orange is used to detect the final end point **Note:** Equivalence point is a point in chemical reaction when chemically equivalent quantities of acid and base are mixed. Methyl orange is a good chemical indicator in strong acid weak base titration as it changes its basic color to acidic color. Methyl Orange shows red color in acidic medium and yellow color in basic medium. In titration, initially $pH$is high, so the solution appears yellow. As acid is added, the solution becomes acidic and methyl orange changes color to red. So color change from yellow to red is an endpoint. Phenolphthalein is used as an indicator for Strong acid -strong base titration. End point is color changes to fuchsia (light pink) color.