What is a standard solution?
A standard solution is a solution whose concentration is accurately known. It is prepared by dissolving a precisely weighed mass of a pure solute and making the solution up to a known volume in a volumetric flask.
Key idea / 要点
mass required (g) = molarity (mol dm−3) × volume (dm3) × molar mass (g mol−1)
Steps to prepare
- Calculate the mass of solute needed and weigh it accurately.
- Dissolve it in a small volume of distilled water in a beaker, stirring with a glass rod.
- Transfer the solution into a volumetric flask using a filter funnel; rinse the beaker and rod into the flask.
- Add distilled water until the bottom of the meniscus sits exactly on the calibration mark.
- Stopper and invert several times to mix thoroughly.
Worked example
Find the mass of sodium carbonate (Na2CO3, M = 106 g mol−1) needed to prepare 250 cm3 of a 0.1 mol dm−3 standard solution.
Moles = 0.1 × 0.25 = 0.025 mol.
Mass = 0.025 × 106 = 2.65 g
Accuracy matters at every step. The solute must be pure and dry so its mass is exactly known, distilled water is used to avoid introducing other ions, and the flask is inverted to make the concentration uniform throughout. When the solute is a stable, pure solid such as sodium carbonate, the solution is called a primary standard and can then be used to standardise other solutions by titration.
Remember
- Use a volumetric flask for accuracy, never a beaker, to fix the final volume.
- Read the meniscus at eye level to avoid parallax error.
- Rinsing the beaker into the flask ensures all solute is transferred.