Page 119 - Chemistry_Form_2

Chemistry
for Secondary Schools

To identify the end point for an acid-base reaction, an indicator must be added.
For example, when phenolphthalein indicator is used in a particular titration (by
running the acid from a burette to a base in a flask), the pink colour of the content
in the flask starts to change slowly to colourless. The end point is marked when
the reaction mixture in the flask just turns colourless. During neutralisation of
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a base with an acid, the volume of the base in a flask is normally known. The
volume of the acid that has neutralised the alkali is called a titre value or titre
volume.
To get the exact volume (V) of the acid that neutralises a base, the initial burette
reading, V is subtracted from the final burette reading V 2 .
1
V = V 2 – V 1

Activity 5.3

Aim: To investigate the neutralisation reaction between sodium hydroxide and
dilute hydrochloric acid

Requirements: Pipette, burette, retort stand and clamp, 10-mL measuring
cylinder, conical flasks, beakers, evaporating dish, white tile,
heat source, tripod stand, wire gauze, 0.1 M sodium hydroxide,
0.1 M hydrochloric acid, and phenolphthalein indicator
Procedure
1. Transfer 25 cm (or 20 cm ) of sodium hydroxide solution into a conical
3
3
flask by using a pipette.
2. Add 2 to 3 drops of phenolphthalein indicator to the solution.

3. Set the apparatus as shown in Figure 5.8.
Burette

Clamp
Acid
Retort
stand

Stopcock

Base
White tile

Figure 5.8: Set-up of the acid-base titration

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CHEMISTRY FORM TWO NEW 2025 DUMMY.indd 112 01/08/2025 11:21:05