Concentration can also be given as the mass of solute in a volume of solution. The unit is grams per cubic decimetre, g/dm3. Concentration (g/dm3) = mass of solute (g) ÷ volume of solution (dm3). A more concentrated solution has more solute in each dm3.
Volumes in the lab are usually in cm3, so divide by 1000 to change to dm3. Example: 5.0 g of sodium chloride is dissolved to make 250 cm3 of solution. Volume = 250 ÷ 1000 = 0.25 dm3. Concentration = 5.0 ÷ 0.25 = 20 g/dm3.
The equation can be rearranged: mass = concentration × volume. So 0.50 dm3 of a 30 g/dm3 solution contains 15 g of solute. Dissolving the same mass in half the volume will double the concentration.
To change a concentration from g/dm3 to mol/dm3, divide by the relative formula mass. For example, 40 g/dm3 of sodium hydroxide is 1.0 mol/dm3 because its relative formula mass is 40.