Disclaimer:
The exact reagents, reactions, processes and equipment used in practical activities may vary between schools, colleges, and exam board exemplar methods. The essential techniques, skills, and learning objectives remain the same. Always follow the instructions, risk assessments, and safety guidance provided by your teacher or centre.
PRACTICAL 1: Preparation of a Volumetric Solution and Acid-Base Titration
Aim:
To accurately prepare a standard solution of sodium hydrogencarbonate and use it to determine the concentration of hydrochloric acid through acid–base titration.
Overview:
The experiment involves dissolving a known mass of sodium hydrogencarbonate to make a volumetric solution, then performing titrations with hydrochloric acid using methyl orange indicator to calculate the acid concentration.
Titration of Sodium Hydrogencarbonate (NaHCO3) with Hydrochloric Acid (HCl)
Apparatus and Chemicals
Apparatus:
- Burette (50 cm3), pipette (25.0 cm3), pipette filler
- Conical flask (250 cm3)
- Volumetric flask (250 cm3)
- Funnel
- White tile
- Wash bottle with distilled water
- Clamp stand and burette clamp
- Glass beaker, stirring rod
Chemicals:
- Sodium hydrogencarbonate (NaHCO3) — solid
- Hydrochloric acid (HCl), approximately 0.1 mol dm−3 (standard solution)
- Methyl Orange indicator
Method
A. Preparing the Standard Solution of Sodium Hydrogencarbonate
Weighing:
- Accurately weigh ~2.10 g of NaHCO3 using a weighing boat and 2 dp balance.
- Transfer to a 250 cm3 beaker.
- Rinse any remaining solid into the beaker with distilled water.
Dissolving:
- Add about 100 cm3 distilled water and stir with a glass rod to dissolve.
Transferring and Diluting:
- Transfer solution to a 250 cm3 volumetric flask using a funnel.
- Rinse the beaker, funnel and glass rod with distilled water and add washings to the flask.
- Fill up to the mark with distilled water using a dropping pipette or clean wash bottle for the final few drops.
- Stopper the flask and invert several times to ensure uniform concentration.
B. Titration Procedure
Setup:
- Rinse burette with HCl and pipette with NaHCO3 solution.
- Fill the burette with HCl and record initial volume.
Measuring NaHCO3:
- Use pipette to transfer 25.0 cm3 of NaHCO3 solution into a conical flask.
- Add 2–3 drops of Methyl Orange indicator.
Titrating:
- Place flask on a white tile.
- Rough titration first - Titrate by adding HCl from the burette while swirling. Add until colour changes from yellow to red.
- Now repeat, however adding HCl dropwise when near rough titration end point, until permanent colour change from yellow to red.
- Record final burette reading and calculate titre.
- Repeat until you have two concordant results within 0.1 cm3.
Example Calculations
Equation:
NaHCO3(aq) + HCl(aq) → NaCl(aq) + CO2(g) + H2O(l)
Suppose average HCl titre = 24.50 cm3
Mass of NaHCO3 used = 2.10 g
- Step 1: Calculate moles of NaHCO3 in 250.0 cm3
Molar mass of NaHCO3 = 84.0 g mol−1
Moles = 2.10 g ÷ 84.0 g mol−1 = 0.02500 mol - Step 2: Calculate concentration of NaHCO3
Volume = 250.0 cm3 = 0.2500 dm3
Concentration = 0.02500 mol ÷ 0.2500 dm3 = 0.100 mol dm−3 - Step 3: Calculate moles of NaHCO3 in 25.0 cm3 sample
Volume = 25.0 cm3 = 0.0250 dm3
Moles = 0.100 mol dm−3 × 0.0250 dm3 = 2.50 × 10−3 mol - Step 4: Moles of HCl (1:1 ratio with NaHCO3)
Moles of HCl = 2.50 × 10−3 mol - Step 5: Average titre = 24.50 cm3 = 0.02450 dm3
Concentration = moles ÷ volume
Concentration of HCl = 2.50 × 10−3 mol ÷ 0.02450 dm3 = 0.102 mol dm−3
Sources of Error and Improvements
- Apparatus error: Burette and pipette tolerances ±0.05 cm3.
- Indicator: End point can be subjective. Use a white tile for clearer view.
- Standard solution: Inaccurate mass or incomplete transfer affects concentration.
- Meniscus error: Always read from the bottom at eye level.
Improvements:
- Use more precise balances for mass.
- Perform multiple repeats for reliability.
Safety and Disposal
- Wear eye protection.
- Hydrochloric acid causes severe skin burns and eye damage
- Sodium hydrogencarbonate is low hazard.