When a solid solute dissolves in water, the mixture is a solution. The concentration of the solution tells you how much solute is dissolved in a given volume of the solution. At GCSE it is measured in grams per cubic decimetre, written g dm−3.
To calculate concentration, divide the mass of solute in grams by the volume of the solution in dm3: concentration = mass ÷ volume. The volume is often given in cm3, so convert it first. There are 1000 cm3 in 1 dm3, so divide the number of cm3 by 1000. For example, 250 cm3 is 0.25 dm3.
Worked example: 12 g of solute is dissolved to make 500 cm3 of solution. The volume is 0.5 dm3, so the concentration is 12 ÷ 0.5 = 24 g dm−3.
The equation can be rearranged. Mass = concentration × volume, and volume = mass ÷ concentration. For example, 250 cm3 of a solution of concentration 40 g dm−3 contains 0.25 × 40 = 10 g of solute.