Molarity is the most common way scientists describe the concentrations of solutions. However, other measurements can be especially useful for solutions that contain complex solutes or multiple solutes, or when the solution is made of two liquids.
Several concentrations are expressed as percentages. Mass percent is the concentration of a solution expressed as the mass of the solute (in grams) divided by the mass of the solvent (in grams), multiplied by 100 to get a percentage. Mass percent is often used in medicine to express the concentration of drugs administered in liquid form. Volume percent is the concentration of a solution expressed as the volume of the solute (in liters) divided by the volume of the solution (in liters), multiplied by 100 to get a percentage. Volume percent is most often used when fluids are mixed, such as the 70% isopropanol that can be purchased in stores. Mass/volume percent is the concentration of a solution expressed as the mass of the solute (in grams) divided by the volume of the solution (in milliliters), multiplied by 100 to get a percentage. It is sometimes called weight/volume percent (w/v). Thus, 15 grams of sodium chloride (NaCl) dissolved in 100 milliliters of water yields a 15% w/v NaCl solution. Mass-volume percent is also often used in medicine to express the concentration of drugs administered in liquid form.
Other concentrations are expressed in terms of the number of units present in the solution. Parts per million (ppm) is concentration expressed as mass of solute divided by mass of solution, multiplied by 106. For an aqueous solution, the parts per million concentration is the mass (in milligrams, mg) of solute per liter of solution. Parts per billion (ppb) is concentration expressed as mass of solute divided by mass of solution, multiplied by 109. For an aqueous solution, the parts per million concentration is the mass (in micrograms, mcg) of solute per liter of solution. Parts per million and parts per billion are useful measurements for non-dissolving solids, such as microorganisms. They are the standard measurements of concentration used by governments to assess very small amounts of contaminants in water, soil, and air.