A solution is made when a solute dissolves in a solvent, usually water. The concentration of a solution tells you how much solute is dissolved in a given volume of solution. A solution with more solute in the same volume is more concentrated.
Concentration is often measured in grams per cubic decimetre (g/dm3): concentration = mass of solute (g) ÷ volume of solution (dm3). Volumes in the lab are often in cm3. One dm3 is 1000 cm3, so divide a volume in cm3 by 1000 to change it into dm3. Rearranged, mass of solute = concentration × volume.
Example: 10 g of solute in 250 cm3 of solution. The volume is 250 ÷ 1000 = 0.25 dm3, so the concentration is 10 ÷ 0.25 = 40 g/dm3. Also, 20 g in 0.5 dm3 gives 40 g/dm3. A solution of 50 g/dm3 holds 50 × 2 = 100 g of solute in 2 dm3. Concentration can also be given in mol/dm3, using moles of solute instead of mass.