By Douglas Bintzler
Updated Aug 30, 2022
Identify whether the concentration is expressed as weight/volume (w/v) or volume/volume (v/v). A w/v solution typically involves a solid dissolved in a liquid, while a v/v solution involves mixing two liquids.
For v/v dilutions, apply the dilution equation C1V1 = C2V2. For example, to prepare 100 mL of 70 % ethanol from 95 % ethanol:
95 % × V1 = 70 % × 100 mL → V1 = 73.6 mL. Add 26.4 mL water to reach 100 mL.
Transfer the liquid solute into a graduated cylinder or volumetric flask before adding solvent. These vessels provide more precise volume readings than beakers, which are better suited for approximate mixing.
For w/v solutions, weigh the solid accurately. A 10 % w/v solution contains 10 g of dry solute in a final volume of 100 mL. Remember that the solute contributes to the total volume.
First dissolve the solid solute in a small volume of solvent to avoid excess solvent usage. Once fully dissolved, transfer the mixture into a graduated cylinder or volumetric flask and add solvent until the desired final volume is reached.
Determine the physical state of the solute. Liquid solutes often come with a specified molarity and may only require dilution. Solid solutes need accurate weighing.
Use the dilution equation for liquid solutes. To dilute 5 M NaCl to 1 M in 100 mL:
5 M × V1 = 1 M × 100 mL → V1 = 20 mL. Add 80 mL water.
Proceed as in Step 3 of the percentage section: pour the liquid solute into a graduated cylinder or volumetric flask, then add solvent to achieve the target volume.
Identify the molar mass (MW) of the solid solute. MW is listed on the container or in the MSDS. For NaCl, MW = 58.4 g/mol, meaning 58.4 g dissolved in 1 L yields a 1 M solution.
Calculate the mass of solute needed for the desired molarity using mass = MW × molarity × volume (L). A 2 M NaCl solution in 1 L requires 58.4 g × 2 = 116.8 g.
Adjust the calculation for the experimental volume. For 100 mL (0.1 L) of 2 M NaCl, the required mass is 0.1 L × 116.8 g = 11.68 g.
Dissolve the solid in a small amount of solvent first, then transfer to a graduated cylinder or volumetric flask and add additional solvent until the final volume is achieved.
Measure the pH with a calibrated pH meter for accuracy; pH paper can be used if a meter is unavailable. For instance, NaCl in water typically yields a neutral pH of 7.
Compare the measured pH to the target value. If the solution is too basic (pH > 7) or too acidic (pH < 7), adjust accordingly.
Add a dilute acid (e.g., 0.1 M HCl) to lower pH or a dilute base (e.g., 0.1 M NaOH) to raise pH. Use small increments to avoid overshooting; these reagents minimally alter the solution’s composition.
Use volumetric flasks for precise volume measurements. Graduated cylinders are acceptable substitutes. Beakers and Erlenmeyer flasks are suitable for mixing but less accurate for final volume.
Always wear appropriate eye protection and, when necessary, a fume hood. Acids, bases, and other chemicals can emit harmful fumes. Consult the MSDS—usually provided with the chemical or available online—for detailed safety instructions.
