Chemists use several different units to express the concentration of a solution, and choosing the right one depends on the context. Molarity is the most common in general chemistry, but molality, mass percent, parts per million, and normality each have their uses. Understanding the differences between these units and knowing when to use each is essential for lab work, research, and passing exams.
Molarity (M)
Molarity is defined as moles of solute per liter of solution. It is the most frequently used concentration unit in chemistry. A 1 M NaCl solution contains 1 mole (58.44 g) of NaCl dissolved in enough water to make exactly 1 liter of solution. Note that molarity depends on the total volume of the solution, not just the volume of the solvent. This means molarity changes with temperature because volume expands when heated. Our Solution Concentration Calculator computes molarity, molality, and mass percent from basic measurements.
Molality (m)
Molality is moles of solute per kilogram of solvent (not solution). Because mass does not change with temperature, molality is temperature-independent, which makes it the preferred unit for colligative property calculations. A 1 m solution of glucose has 1 mole of glucose dissolved in 1 kg (about 1 liter) of water. For dilute aqueous solutions, molarity and molality are nearly equal, but they diverge at higher concentrations.
Mass Percent and ppm
Mass percent is (mass of solute / mass of solution) x 100%. It is commonly used in industry and commerce. A 3% mass percent hydrogen peroxide solution, for example, contains 3 g of H2O2 per 100 g of solution. Parts per million (ppm) is used for very dilute solutions and is equivalent to mg/L for aqueous solutions. Drinking water standards often express contaminant limits in ppm. A concentration of 1 ppm means 1 mg of solute per liter of water, or roughly one drop of ink in a large swimming pool.