Question

Assertion: In $NaHC{{O}_{3}}$ solution, phenolphthalein is colourless.
Reason: Phenolphthalein is colourless in basic medium and pH of $NaHC{{O}_{3}}$ is less than 10.
[A] Both assertion and reason are correct and the reason is the correct explanation for assertion.
[B] Both assertion and reason are correct but the reason is not the correct explanation for the assertion.
[C] Assertion is correct but reason is incorrect.
[D] Both assertion and reason are incorrect.

Answer 1

Hint: To answer this you must know that phenolphthalein is a weak acid and is coloured in highly basic solutions. It is colourless in basic solution but shows pink colouration in solutions above pH of 9. You can use this to answer the given question.

Complete answer:
Before answering this question, let us understand what an indicator is and how the indicator given to us i.e. phenolphthalein works.
We know that in chemistry, we use indicators during titration to mark the end point of a titration. We see that in acid-base titrations, generally the end is marked by the change in colour of the solution.
We use indicators for the determination of the pH of a solution. Different indicators work differently in acidic, basic and neutral solutions.
Depending upon the titration, it is very important for us to choose suitable indicators which will indicate the equivalence point.
Now let us discuss phenolphthalein.
Phenolphthalein is an organic compound with the formula ${{C}_{20}}{{H}_{14}}{{O}_{4}}$. It is a weak acid and below the pH of 8.5, it is colourless and addition of hydroxyl ions to the solution i.e. increasing the pH of the solution turns the colour to pink.
In the question, the assertion given to us is -$NaHC{{O}_{3}}$ solution, phenolphthalein is colourless.
The compound given to us is sodium bicarbonate and its pH is slightly above 7. As we have discussed above that phenolphthalein is colourless in solutions having pH under 8.5. Therefore, it is colourless in $NaHC{{O}_{3}}$ and thus the assertion is correct.
And the reason given is - Phenolphthalein is colourless in basic medium and pH of $NaHC{{O}_{3}}$ is less than 10.
We have already discussed that phenolphthalein is pink coloured in basic medium. Therefore, the reason is incorrect. However, the second part of the reason i.e. pH of $NaHC{{O}_{3}}$ is less than 10 is correct.

Therefore, the correct answer is option [C] assertion is correct but reason is incorrect.

Note:
During a titration, the equivalence point is basically the point of 100% neutralisation. The end point of a titration in presence of a given indicator is marked by a characteristic change.
However, the end point doesn’t actually coincide with the equivalence point. Due to this the solution is somewhat over-titrated or under-titrated which gives rise to small errors during titration. This is called indicator error.