Preparing a Standard Solution

Preparing a Standard Solution

Preparing a standard solution is a crucial laboratory skill that involves accurately creating a solution with a known concentration. Here's a step-by-step guide for secondary school students:

Step 1: Calculate the Required Mass

• Calculate the mass of the substance (solute) needed to make the standard solution.
  • Number of moles of solute (n) = Concentration (c) × Volume (v)
• Example: If you need 250 cm³ of a 0.25 mol/L ammonium sulphate solution:
  • n = 0.25 mol/L × 0.25 L = 0.0625 moles
  • The formula of ammonium sulphate: (NH₄)₂SO₄
  • Calculate the gram formula mass (gfm) of ammonium sulphate: (2 × 14) + (8 × 1) + (32.1 × 1) + (16 × 4) = 132.1 g/mol
  • Mass of ammonium sulphate required = n × gfm = 0.0625 moles × 132.1 g/mol = 8.256 g

Step 2: Making the Standard Solution

• Stage 1: Accurate Mass Measurement
  • Measure 8.256 g of ammonium sulphate accurately using a balance.
• Stage 2: Dissolving the Solute
  • Dissolve the measured solid ammonium sulphate in a small quantity of deionised water in a beaker.
• Stage 3: Transferring to a Standard Flask
  • Once the solid has fully dissolved, carefully transfer the solution into a standard flask.
  • Rinse the beaker with deionised water several times to ensure all the solution is transferred to the flask.
• Stage 4: Adding Water to the Mark
  • Add deionised water to the standard flask until it reaches the mark on the flask.
  • Ensure the bottom of the meniscus touches the mark for precise volume.
• Stage 5: Mixing the Solution
  • Stopper the flask and invert it multiple times to thoroughly mix the solution.

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Preparing a Standard Solution

Additional Tips

• Do not use tap water, as it contains dissolved salts that can react with the solute and affect the solution's concentration.
• Deionised water, which has had ions removed, should be used to make standard solutions for accuracy.

Conclusion:

• Preparing a standard solution involves precise measurement, dissolution, transfer, dilution, and thorough mixing.
• Standard solutions are crucial for accurate chemical analyses and experiments in the laboratory, as they provide known concentrations for calibration and reference purposes.