Page 27 - Chemistry - XI

Precautions
1. Equal number of drops of each solution should be dropped on pH paper or universal indicator.
2. Salt added to the solution must dissolve completely.
3. Match the colour carefully with the pH chart.

VIVA VOCE

Q1. What is common ion eff ect?
Ans. The decrease in concentration of one of the ions by adding other ions as common ions is called
common ion eff ect.

Q2. What is buff er solution?
Ans. A solution whose pH does not change by adding a small amount of acid or base is called buff er
solution.

Q3. How does the pH of acetic acid change on addition of sodium acetate to it?
Ans. It will increase.

Q4. How does pH of NH OH change on addition of NH Cl to it?
Ans. It will decrease. 4 4

EXPERIMENT - 12

Objective
To study the pH change in the titration of a strong base with a strong acid using universal indicator.

Principle/Theory
Titration of a strong acid and a strong base involves a neutralisation reaction, and a salt and water is formed.

NaOH + HCl NaCl + H O
2
If we take NaOH in a titration fl ask, its initial pH will be greater than 7, pH will keep on decreasing on
addition of acid from the burette. At neutralisation point (end point), pH of solution becomes 7 because
strong acid reacts completely with strong base to form salt which is neutral and cannot be hydrolysed.

Materials Required
Burette (50 mL), pipette (25 mL), conical fl ask (250 mL), burette stand, funnel, white glazed tile, universal

indicator, N/10 solution of NaOH and N/10 solution of HCl

Procedure
1. Take 10 mL of N/10 NaOH with the help of a burette in a conical fl ask.
2. Add 2 to 3 drops of universal indicator and observe the colour change and note down the pH.

3. Add 2.5 mL of N/10 HCl from the burette slowly by adding small amount at a time with constant
shaking.
4. Add 2 to 3 drops of universal indicator and observe the colour change and note down the pH.
Add 2 to 3 drops of universal indicator and observe the colour change and note down the pH.

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