Core Practical 3 – Find the Concentration of Hydrochloric Acid

Chemical Equation

Balanced Equation:
HCl + NaOH → NaCl + H₂O

This is a 1:1 molar ratio reaction between a strong acid and a strong base.

Apparatus and Chemicals

Equipment:

• Burette, clamp and stand
• 25 cm³ pipette and safety filler
• 250 cm³ volumetric flask
• 250 cm³ conical flask
• 100 cm³ and 250 cm³ beakers
• Funnel
• Distilled and deionised water

Chemicals:

• Standard sodium hydroxide solution (~0.08 mol dm⁻³)
• Hydrochloric acid of unknown concentration
• Phenolphthalein indicator

Safety Notes

• Wear goggles throughout the experiment.
• Phenolphthalein is flammable and toxic.
• Hydrochloric acid is an irritant – avoid contact with skin.
• Fill burettes below eye level to avoid splashes.

Procedure

Prepare the solution for titration:

• Wash out a 250 cm³ volumetric flask with distilled water.
• Use a pipette to transfer exactly 25.0 cm³ of the unknown HCl solution into the flask.
• Fill the flask to the mark with deionised water – this dilutes the acid to a known volume.

Set up titration equipment:

• Fill a burette with standard sodium hydroxide solution.
• Place 25.0 cm³ of the diluted hydrochloric acid in a conical flask using a clean pipette.

Add indicator:

• Add 2 drops of phenolphthalein – the solution should remain colourless (acidic).

Titrate:

• Slowly add NaOH from the burette while swirling.
• The endpoint is reached when the solution turns pale pink and stays pink for at least 5 seconds.
• Record the initial and final burette readings to the nearest 0.05 cm³.
• Repeat until you obtain two concordant titres (within 0.10 cm³).
• Record results in a table.

Sample Data

Analysis of Results

• Mean titre (use concordant values only)
  Example: Mean = (30.05 + 29.95) / 2 = 30.00 cm³
• Moles of NaOH in mean titre
  Suppose [NaOH] = 0.080 mol dm⁻³
  Volume = 30.00 cm³ = 0.03000 dm³
  Moles = 0.080 × 0.03000 = 0.00240 mol
• Moles of HCl in 25.0 cm³ aliquot
  Ratio is 1:1, so moles HCl = 0.00240 mol
• Moles in full 250 cm³ solution
  = 0.00240 × 10 = 0.0240 mol
• Concentration of original HCl
  c = n / V = 0.0240 / 0.025 = 0.96 mol dm⁻³

Matt’s Exam Tip

Do not include the rough titration when calculating your mean.
Record all burette readings to the nearest 0.05 cm³.
Consistent swirling between adding drops from burette prevents overshooting the endpoint.

Common Errors and Considerations

• Phenolphthalein may fade after turning pink due to reaction with atmospheric CO₂ (forming carbonic acid).
• Best titre volume is around 25–30 cm³; too small and percentage errors increase.