A stock solution is prepared by weighing out an appropriate portion of a pure solid or by measuring out an appropriate volume of a pure liquid and diluting to a known volume. Exactly how this is done depends on the required concentration units. For example, to prepare a solution with a desired molarity you would weigh out an appropriate mass of the reagent, dissolve it in a portion of solvent, and bring to the desired volume. To prepare a solution where the solute’s concentration is given as a volume percent, you would measure out an appropriate volume of solute and add sufficient solvent to obtain the desired total volume.
Mass, molar concentration, volume, and formula weight are related to each other as follows:
Mass (g) = Concentration (mol/L) * Volume (L) * Formula Weight (g/mol)
For example, if we want to prepare 500 mL of approximately 0.20 mol/L NaOH using solid NaOH. Since the concentration only needs to be known to two significant figures, the mass of NaOH and volume of solution do not need to be measured exactly. The desired mass of NaOH is
Mass= 0.2 mol/L* 0.5 L* 40 g/mol= 4 g
To prepare the solution we place 4.0 g of NaOH, weighed to the nearest tenth of a gram, in a bottle or beaker and add approximately 500 mL of water.