A solution forms when a solute dissolves in a solvent. The concentration of a solution tells you how much solute is dissolved in a certain volume of the solution. At GCSE it is measured in grams per cubic decimetre, written g/dm3.
concentration (g/dm3) = mass of solute (g) ÷ volume of solution (dm3)
Volumes are often given in cm3, but the equation needs dm3. Since 1 dm3 is 1000 cm3, you divide a volume in cm3 by 1000. For example, 250 cm3 is 0.25 dm3.
Worked example: 20 g of solute is dissolved to make 500 cm3 of solution. The volume is 0.5 dm3, so the concentration is 20 ÷ 0.5 = 40 g/dm3. The equation can be rearranged to give mass = concentration × volume. A 15 g/dm3 solution therefore holds 15 × 2 = 30 g of solute in 2 dm3.