C = n / V (molarity) or C = m / V (mass concentration)
Concentration describes how much solute is dissolved in a given amount of solvent or solution. The most common unit in chemistry is molarity (M) = moles of solute per liter of solution. Mass concentration (g/L) is mass of solute per liter. Percent concentration can be weight/weight (w/w%), weight/volume (w/v%), or volume/volume (v/v%). Converting between units requires knowing the molar mass of the solute. For example, 1 M NaCl = 58.44 g/L NaCl. Concentration affects reaction rates (rate law), equilibrium positions, colligative properties (boiling point elevation, freezing point depression, osmotic pressure), and is critical in medical dosing, environmental monitoring, and industrial chemistry.