Acid–base titration: finding an unknown concentration

Objectives

• Set up and use a burette.
• Detect an end point with phenolphthalein.
• Calculate concentration from a titre.

Theory

In a titration a solution of known concentration is run from a burette into a measured volume of the other solution until the reaction is just complete. For HCl + NaOH → NaCl + H₂O the mole ratio is 1 : 1, so M(acid) × V(acid) = M(base) × V(base).

HCl + NaOH → NaCl + H₂O

Procedure

1. Put on your safety goggles and lab coat.
2. Place a conical flask and a burette on the bench.
3. Add exactly 25 mL of the acid sample to the conical flask.
4. Add a few drops of phenolphthalein to the flask.
5. Fill the burette with 0.100 M sodium hydroxide.
6. Move the conical flask under the burette.
7. Run in sodium hydroxide until the solution just turns pale pink.
8. Read the burette and note the volume delivered.

Precautions

• Fill the burette below eye level.
• Add the alkali dropwise near the end point.
• Read the burette to 0.05 mL.

Subject

Chemistry

Grade

10–12

Chapter

Volumetric Analysis (Nepal National Curriculum (CDC), grade 11)

Difficulty

Medium

Duration

25 min

Safety level

Medium

Version

1.0

Apparatus

• Conical flask (250 mL)
• Burette on stand (50 mL)

Chemicals

• Hydrochloric acid sample (unknown concentration) HCl Corrosive
• Sodium hydroxide 0.100 M NaOH Corrosive
• Phenolphthalein indicator C₂₀H₁₄O₄