The concentration of a solution tells you how much solute is dissolved in a given volume of solution. In OCR chemistry it is measured in moles per cubic decimetre, written mol/dm3. A solution with a concentration of 1 mol/dm3 has one mole of solute in every 1 dm3 of solution.
Concentration (mol/dm3) = amount of solute (mol) ÷ volume of solution (dm3). Volumes are often measured in cm3, so divide by 1000 to convert to dm3, because 1 dm3 = 1000 cm3.
The amount of solute usually has to be worked out from a mass first. Use moles = mass ÷ relative formula mass (Mr), then divide the moles by the volume.
Example: 4.0 g of sodium hydroxide, NaOH (Mr = 40), is dissolved to make 500 cm3 of solution. Moles = 4.0 ÷ 40 = 0.10 mol. Volume = 500 ÷ 1000 = 0.50 dm3. Concentration = 0.10 ÷ 0.50 = 0.20 mol/dm3. The equation can be rearranged: moles = concentration × volume, and mass = moles × Mr.