Core Practical 2: Find the Concentration of a Solution of Sodium Hydroxide

Chemical Background

Balanced Equation:

NH₂SO₃H (s) + NaOH → NaSO₃NH₂ + H₂O

Sulfamic acid is monoprotic, meaning one mole reacts with one mole of NaOH.

Apparatus and Chemicals

Equipment

• Mass balance (2 d.p.)
• 250 cm³ volumetric flask
• Burette, pipette (25 cm³), pipette filler
• Beakers, 250 cm³ conical flask
• Funnel for burette filling
• Clamp and stand
• Goggles

Chemicals

• ~2.5 g solid sulfamic acid
• 200 cm³ sodium hydroxide solution of unknown concentration (~0.08 mol dm⁻³)
• Methyl orange indicator

Safety Notes

• Wear eye protection throughout.
• Sulfamic acid is toxic if ingested.
• Fill burette below eye level to avoid splashes.

Procedure

1. Weigh an empty test tube.
2. Add ~2.5 g sulfamic acid and reweigh to obtain the exact mass.
3. Dissolve in ~100 cm³ of water in a beaker.
4. Transfer solution and washings into a 250 cm³ volumetric flask and top up with deionised water.

This produces a standard solution of sulfamic acid.

Titration setup:

• Burette: sulfamic acid solution
• Conical flask: 25.0 cm³ sodium hydroxide + 4 drops methyl orange

Titrate carefully, swirling constantly. Near the endpoint, add dropwise until a permanent colour change. Record to 0.05 cm³. Repeat to obtain concordant titres (within 0.1 cm³).

Sample Data

Mass of sulfamic acid = 2.41 g
M_r = 97.1
Moles in 250 cm³ = 2.41 ÷ 97.1 = 0.0248 mol
Concentration = 0.0248 ÷ 0.250 = 0.099 mol dm⁻³

Concordant titres: 20.05 and 20.05 cm³
Mean titre: 20.05 cm³

Calculations

• Volume of sulfamic acid titre = 0.02005 dm³
• Moles = 0.099 × 0.02005 = 0.001985 mol
• Moles NaOH = 0.001985 (1:1 ratio)
• Volume NaOH = 25.0 cm³ = 0.025 dm³
• Concentration NaOH = 0.001985 ÷ 0.025 = 0.0794 mol dm⁻³

Common Errors and Improvements

• Error: Water left in pipette could dilute NaOH. Fix: Rinse pipette with NaOH.
• Error: Reading burette from above eye level. Fix: Read at eye level.
• Error: Inconsistent swirling. Fix: Swirl consistently throughout.

Matt’s Exam Tip

Always rinse the pipette and burette with the solution it will transfer. Only use a few drops of indicator — excess can affect the endpoint.