## 4. Making solutions of known concentration

Concentrations of solutions expressed on a mass per unit volume basis (eg mg l^{-1} or mmol m^{-3}) require knowledge of the mass of solute added and the volume of the solution. To make a solution of known concentration, a specified quantity of the solute is weighed out accurately. This is added to a specially calibrated volumetric flask, and solvent is added to make an exact volume of solution. An example:

To make 250 ml of a 8.5 g l^{-1} solution of sodium nitrate, first calculate the amount of solute needed for the volume of solution. The volumetric flask contains 250 ml = 0.25 l. If a litre of solution contains 8.5 g, 250 ml will contain a quarter of that amount of solute. So the mass of solute needed to make the solution will be 0.25 x 8.5 = 2.125 g.

Dissolving 2.125 g of sodium nitrate in 250 ml of water gives a solution of 8.5 g l^{-1}, which can be expressed in terms of molarity (see previous section) as 0.1 mol l^{-1}.