Page 26 - Chemistry - XI

EXPERIMENT - 11

Objective
To study the variation in pH by common ion eff ect in the case of weak acid (acetic acid) and weak base

(NH OH).
4
Principle/Theory
When to a solution of weak acid or weak base, its salt containing its common ion is added, the pH of solution
increases, e.g. when sodium acetate is added to acetic acid solution, concentration of CH COO increases
–
3
and therefore concentration of H O decreases which ultimately increases the pH.
+
3

CH COOH HO CH COO H O

3 2 3 3

CH COONa CH COO Na
3 3
This solution is an example of acidic buff er. Its pH can be calculated by the following equation:
pH pK log [ Salt]
a
[ Acid]
where pK = – log K a
a
When to a solution of weak base like NH OH, salt like NH Cl is added, NH ion being common ion,
+
4
4
4
concentration of OH decreases whereas concentration of H O increases, consequently pH will decrease.
–
+
3
NH OH NH OH

4 4

NH Cl NH Cl
4 4
pOH pK log [ Salt]
b
[ Base]
pH = 14 – pOH
Materials Required
Boiling tubes, test tube stand, dropper and glass rod, acetic acid, sodium acetate, NH Cl, NH OH, pH
4
4
paper, universal indicator, and distilled water
Procedure

1. Take fi ve marked boiling tubes A, B, C, D and E.
2. In each of these, add 1 mL of acetic acid and 10 mL of distilled water and shake the solution well.
3. Put a drop of this solution on the pH paper or universal indicator and match the colour so produced with
the standard pH chart and note down the pH of the acetic acid.
4. Now, add 0.5 g, 1.0 g, 2.0 g and 3.0 g of powdered sodium acetate in the boiling tubes B, C, D and
E respectively and shake the solutions well.

5. Determine the pH of the above solutions by using pH paper and note down your observations in a
tabular form and draw your conclusions.
6.
6. Similarly, experiment may be performed by adding NH Cl solid in the solution of NH OH (weak base) Similarly, experiment may be performed by adding NH 4 4
and pH value may be recorded in the tabular form and conclusion may be drawn.and pH value may be recorded in the tabular form and conclusion may be drawn.

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