The concentration of a solution tells you how much solute is dissolved in each cubic decimetre. In mol/dm3 it is the number of moles divided by the volume in dm3. Volumes are usually measured in cm3, so first convert to dm3 by dividing by 1000.
Example: 4.0 g of sodium hydroxide, NaOH (relative formula mass 40), is dissolved to make 500 cm3 of solution. Moles = 4.0 ÷ 40 = 0.10 mol. Volume = 500 ÷ 1000 = 0.500 dm3. Concentration = 0.10 ÷ 0.500 = 0.20 mol/dm3.
A concentration can also be given as a mass per volume in g/dm3. To change g/dm3 into mol/dm3, divide by the relative formula mass. To change mol/dm3 into g/dm3, multiply by the relative formula mass. For hydrochloric acid, HCl (relative formula mass 36.5), a solution of 36.5 g/dm3 is 1.0 mol/dm3. A solution twice as concentrated, 2.0 mol/dm3, contains 73 g/dm3.