Molarity is the number of moles of solute dissolved in one liter of solution . Molarity is usually denoted with a capital M, i.e. a 0.50 M solution. Also recognize that molarity changes slightly with temperature because the volume of a solution changes with temperature. Molality is the number of moles of solute dissolved in one kilogram of solvent. Notice the two key differences between molarity and molality.
Molality uses mass rather than volume and uses solvent instead of solution.
Unlike molarity, molality is independent of temperature because mass does not change with temperature. Molality is usually denoted with a small m, i.e. a 0.50 m solution. Parts per million works like percent by mass, but is more convenient when there is only a small amount of solute present. A solution with a concentration of 1 ppm has 1 gram of substance for every million grams of solution. Therefore, in general, one ppm implies one mg of solute per liter of solution.