Question

Your teacher uses a weighing balance to take equal amounts of two substances, tartaric acid and washing soda, say 1g. Each is dissolved separately into 100 cc of water.

(A) In 1 drop of the acid solution and 1 drop of the basic solution, we have:
1. equal amount of acid and base respectively
2. equally acidic and basic substance respectively
3. acidity in one and basicity in the other are not equal
4. equal magnitude of the quantity —pH-7—
(B) Take a few cc of the acidic solution in a test-tube and mix a few drops of coloured
phenolphthaline solution (prepared in basic medium) into it. Which of the following may be 4
happening:
1. The colour of the solution instantly changes pink
2. remains colourless as the colour of the added drops disappears
3. the colour diffuses through the solution and finally disappears
4. the colour diffuses through the solution and finally the entire solution acquires a faint pink
colour.

• (A) 3, (B) 4
• (A) 3, (B) 3

Hint:

Phenolphthalein is a pH indicator that is colourless in acidic solutions and turns pink in basic solutions (pH>8.4).

Answer 1

The Correct Option is A

Solution: In the given case, the amount of acid and base may not necessarily be equal as
the strength (pH) of the acid and base solutions are different. Therefore, the acidity in one
and the basicity in the other are not equal. Hence, option 3 is correct. (B) When a few drops
of phenolphthalein are added to an acidic solution, the colour will remain colourless as the
solution is acidic and will not turn pink until the pH reaches around 8.4, which is the pH at
which phenolphthalein changes colour. Hence, option 4 is correct.

Correct Answer: (A) 3, (B) 4