Many reactions happen in solutions. The concentration of a solution tells you how much solute is dissolved in a given volume of solution. It can be measured as a mass per volume, for example grams per dm3 (g/dm3).
concentration (g/dm3) = mass of solute (g) ÷ volume of solution (dm3). Rearranged, mass of solute = concentration × volume. There are 1000 cm3 in 1 dm3, so to convert a volume from cm3 to dm3 you divide by 1000. For example, 250 cm3 is 0.25 dm3.
Example: a solution has a concentration of 20 g/dm3. The mass of solute in 250 cm3 is 20 × 0.25 = 5 g.
The mass of solute and the volume of solution control the concentration. If the same mass of solute is dissolved in a smaller volume, the concentration is higher. If twice the mass is dissolved in the same volume, the concentration doubles. Adding water to dilute a solution increases the volume and decreases the concentration.