Many reactions take place in solution. 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 in grams per cubic decimetre (g/dm3). The calculation is: concentration = mass of solute ÷ volume of solution.
Volumes are often given in cm3, so convert first. There are 1000 cm3 in 1 dm3, so divide cm3 by 1000 to get dm3. For example, 250 cm3 is 0.25 dm3. If 10 g of salt is dissolved to make 250 cm3 of solution, the concentration is 10 ÷ 0.25 = 40 g/dm3.
Rearranged, mass of solute = concentration × volume. A 500 cm3 sample is 0.5 dm3. If its concentration is 30 g/dm3, it contains 30 × 0.5 = 15 g of solute.
Higher tier: concentration depends on both the mass of solute and the volume. Doubling the mass of solute in the same volume doubles the concentration. Doubling the volume with the same mass of solute means the concentration is halved. Dissolving the same mass in a smaller volume gives a higher concentration.