A standard solution is a solution whose concentration is known accurately, and it is essential for titrations, calibration curves, and quantitative chemical analysis. Preparing one correctly depends on careful mass measurement, complete dissolving, and precise dilution in a volumetric flask. Small errors in weighing, transferring, or reading the meniscus can cause the final molarity to be wrong.
The goal is to make a solution with a known amount of solute in a known final volume.
The usual method is to weigh a suitable solute, dissolve it in a small amount of solvent, transfer it quantitatively to a volumetric flask, and dilute to the calibration mark. A primary standard is often used because it is pure, stable, and can be weighed accurately. The volumetric flask is designed so that the volume is correct only when the bottom of the meniscus sits exactly on the calibration line at the stated temperature.
Molarity is calculated from moles of solute divided by liters of solution, so both the chemical amount and final volume must be controlled.
Key Facts
- Molarity is concentration in moles per liter: M = n / V.
- Moles from mass are calculated by n = m / M_m, where M_m is molar mass.
- To prepare a target solution, use m = M × V × M_m.
- A volumetric flask gives an accurate final volume only when filled to its calibration line.
- Read the meniscus at eye level using the bottom of the curve for most aqueous solutions.
- Quantitative transfer means all solute and rinse liquid must end up in the volumetric flask.
Vocabulary
- Standard solution
- A solution with a concentration that is known accurately for use in quantitative experiments.
- Volumetric flask
- A glass flask calibrated to contain one precise volume when filled to its mark.
- Primary standard
- A highly pure, stable substance that can be weighed directly to prepare a solution of known concentration.
- Meniscus
- The curved surface of a liquid in a container, used to judge the correct volume reading.
- Molarity
- The number of moles of solute dissolved per liter of solution.
Common Mistakes to Avoid
- Filling past the calibration mark, which makes the solution too dilute because the final volume is larger than intended.
- Reading the meniscus from above or below eye level, which causes parallax error and gives an inaccurate final volume.
- Adding solvent to the mark before the solute is fully dissolved, which can leave undissolved solid and make the concentration too low.
- Losing solid during transfer, which reduces the number of moles in the flask and makes the prepared solution less concentrated than calculated.
Practice Questions
- 1 How many grams of NaCl are needed to prepare 250.0 mL of 0.100 M NaCl solution? Use molar mass NaCl = 58.44 g/mol.
- 2 A student dissolves 2.65 g of Na2CO3 and dilutes the solution to 500.0 mL. What is the molarity? Use molar mass Na2CO3 = 105.99 g/mol.
- 3 A student weighs the correct mass of a primary standard but leaves a few crystals on the weighing paper during transfer. Explain how this affects the final concentration and why.