Preparing a solution: by dissolution or by dilution

Two methods, two starting points

There are two main ways to prepare a solution of a given concentration: starting with a solid solute (or pure liquid) and dissolving it, or starting with an existing solution and diluting it.

Method 1: Preparation by dissolution

We wish to prepare a volume V of a solution of known concentration c.

1. We calculate the amount of substance required: n = c × V
2. We calculate the corresponding mass of solute: m = n × M
3. We weigh this mass m of solute, pour it into a volumetric flask, add water to dissolve it, then make up to the mark (to the desired volume V).

Example: prepare 100 mL (0.100 L) of a NaCl solution with c = 0.50 mol/L (M(NaCl) = 58.5 g/mol):

n = c x V = 0,50 x 0,100 = 0,050 mol
m = n x M = 0,050 x 58,5 = 2,925 g

Method 2: Preparation by dilution

We wish to prepare a volume V2 of a solution of concentration C2 from a stock solution of concentration C1.

1. Calculate the volume to be taken: V1 = (C2 × V2) / C1
2. Pipette this volume V1 (often using a graduated pipette), transfer it to a volumetric flask of volume V2, then make up to the mark with water.

Common mistake

A frequent mistake is to add all the solvent first, followed by the solute, without following the correct safety procedure (particularly with concentrated acids, where the acid is always added to water, never the other way round). Another mistake is forgetting to make up to the exact mark: the final volume must be precise for the calculated concentration to be correct.