Edexcel International A Level (IAL) Chemistry (YCH11) - Unit 2 - 8.21 CORE PRACTICAL 3: HCl concentration-Study Notes - New Syllabus

8.21 CORE PRACTICAL 3 – Finding the Concentration of Hydrochloric Acid

This practical determines the concentration of \( \mathrm{HCl} \) using a standard solution (usually \( \mathrm{Na_2CO_3} \) or \( \mathrm{NaOH} \)) by titration.

Principle

Unknown acid concentration is found using a reaction with known concentration.

Typical Reaction (with Sodium Carbonate)

\( \mathrm{Na_2CO_3 + 2HCl \rightarrow 2NaCl + H_2O + CO_2} \)

Apparatus

• Burette (with \( \mathrm{HCl} \))
• Pipette (25.0 cm\(^3\))
• Conical flask
• Indicator (methyl orange)
• Standard solution (e.g. \( \mathrm{Na_2CO_3} \))

Procedure

• Use pipette to transfer 25.0 cm\(^3\) of standard solution into flask.
• Add a few drops of methyl orange.
• Fill burette with \( \mathrm{HCl} \).
• Add acid slowly while swirling.
• Stop at end point (colour change).
• Record titre. Repeat for concordant results.

Indicator Choice

• Methyl orange: yellow → orange/red
• Suitable for acid–carbonate reaction

Calculations

• Step 1: Calculate moles of standard solution
• Step 2: Use mole ratio (from equation)
• Step 3: Find moles of \( \mathrm{HCl} \)
• Step 4: Use \( \mathrm{c = \frac{n}{V}} \)

Key Experimental Skills

• Read burette at eye level (±0.05 cm\(^3\))
• Rinse apparatus with correct solutions
• Swirl continuously
• Add acid dropwise near endpoint

Sources of Error

• Overshooting endpoint
• Parallax error in reading burette
• Incomplete mixing

Summary

• Use titration to determine unknown \( \mathrm{HCl} \) concentration.
• Requires accurate measurements and correct indicator.
• Based on stoichiometry and \( \mathrm{n = cV} \).

Therefore, this practical links experimental technique with quantitative chemical analysis.