Question

In the titration of strong acid and weak base, the indicator used is:
(A)-Thymol Blue
(B)- phenolphthalein
(C)-Thymolphthalein
(D)- Methyl Orange

Answer 1

Hint: Strong acid reacts with weak base; this will result in an acidic solution so $pH$ will be less than 7.
When strong acid is titrated into a weak base, $pH$ of base is high , as acid is added, $pH$ will change gradually. In this titration, Indicator that changes its basic color to acidic color is a good choice.

Complete answer:
-In this titration, Strong acid is titrated into a weak base. Burette is filled with a small amount of acid of known concentration. A base of unknown concentration and of known volume is taken into a flask.
-Titration is based on the principle of neutralization.
When strong acid reacts with strong base, the final solution is neutral and has $pH$ 7 because ${{H}^{+}}$ions of acid are neutralized completely $O{{H}^{-}}$ ions of base.
-Strong acid completely dissociates in water and weak base does not dissociate completely.
Initially $pH$ of weak base is high, as acid is added dropwise, $pH$ gradually changes. Finally one drop of acid results in rapid change in $pH$ when an equivalent point is reached.
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.
In the titration of strong acid and weak base, the indicator used is Methyl Orange.

Note:
Phenolphthalein is used as an indicator for Strong acid -strong base titration. End point is color changes to fuchsia (light pink) color. Thymolphthalein is used as an indicator for weak acid and strong base. The end point is color change to blue.