A solution is composed of a relatively small number of solute molecules or ions dissolved in a much larger number of solvent molecules. Vinegar, for example, is a solution of about 5% acetic acid molecules dissolved in about 95% water molecules.
The amount of solute in a solution is most often described in terms of the concentration of the solute in a given amount of solution. Most commonly, this is given as molarity — the moles of solute divided by the liters of solution:
Molarity is often denoted with a capital M. So a solution described as "3.6 M HCl" has a concentration of 3.6 mol HCl per liter of solution. You will also see molarity indicated by putting the solute's formula in square brackets, as in [HCl] = 3.6 M.
Notice that the same amount of solute gives a higher concentration in a smaller volume of solution, and a lower concentration in a larger volume.
Three levels on determining and using solution concentration. Pick a level to begin.