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 ignore the safety order when diluting a concentrated acid: the acid is always added to the water, never the water to the acid. Another mistake is forgetting to make up to the exact mark: the final volume must be precise for the calculated concentration to be correct.