Question

Sara took 2 mL of dilute NaOH solution in a test tube and added two drops of phenolphthalein solution to it. The solution turned pink in colour. She added dilute H₂SO₄ to the above solution drop by drop until the solution in the test tube became colourless. 40 drops of dilute H₂SO₄ were used for the change in colour from pink to colourless. When Sara added a drop of NaOH to the solution, the colour changed to back to pink again.

Sara now tried the activity with different volumes of NaOH and recorded her observation in the table given below:

S. No.	Volume of dil. NaOH taken (mL)	Drops of dil. H2SO4 used
1	2	20
2	3	30
3	4	40

Answer the following questions based on the above information:

A. If Sara used concentrated H₂SO₄ in place of dilute H₂SO₄, how many drops will be required for the change in colour to be observed?

1. 40
2. < 40
3. > 40

Justify your answer.

B. Sara measured 20 drops of dil. H₂SO₄ and found its volume to be 1 mL. If Sara observed a change in colour of NaOH solution by using 3 mL of H₂SO₄, how many mL of NaOH did she add to the test tube initially?

OR

Sara takes 10 drops of dilute H₂SO₄ in the test tube and adds two drops of phenolphthalein solution to it. Then she adds NaOH dropwise. Sara observes a change in colour after adding 20 drops of NaOH. What change in colour would she observe and why?

C. Write a balanced chemical equation for the reaction taking place in the above experiment. Which of the following is true and why? The reaction is:

1. neutralisation and double displacement reaction
2. neutralisation and precipitation reaction
3. precipitation and double displacement reaction
4. neutralisation, double displacement as well as precipitation reaction.

Answer 1

A. < 40

Justification: Concentrated H₂SO₄ has more H⁺ ions per drop than dilute H₂SO₄, so fewer drops are needed to bring about the colour change. Therefore, less than 40 drops will be required.

B. 3 mL of H₂SO₄ will be 60 drops, which will neutralise 6 mL of NaOH.

S. No.	Volume of dil. NaOH taken (mL)	Drops of dil. H2SO4 used
1	2	20 (1 mL)
2	3	30 (1.5 mL)
3	4	40 (2 mL)
4	6	3 mL = 60 drops

OR

Colour will change from colourless to pink. Phenolphthalein in colourless in acids and turns pink in basic solution.

C. \[\ce{2NaOH + H2SO4 -> Na2SO4 + 2H2O}\]

Neutralisation and double displacement reaction.

Base NaOH is getting neutralised and forming salt + water. It is double displacement, as Na⁺ ions are being replaced by H⁺ and OH⁻ by \[\ce{SO^{2-}_4}\]. It is not a precipitation reaction because Na₂SO₄ is soluble in water.