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Molality is a ratio used in analytical chemistry to quantitatively express the concentration of a chemical solution. When a solution is made, it contains a certain amount of solute, a substance that is dissolved into a liquid. The liquid into which the solute is dissolved is known as the solvent. Molality (m) is the amount of solute, expressed in units called moles, divided by the mass of the solvent in kilograms.
In order to understand molality, it is necessary to understand the mole, a fundamental chemical unit. One mole of a substance is defined as containing the same number of units as the number of atoms in 12 grams (g) of the element carbon-12. This very high number is known as Avogadro's Number. A mole of carbon-12, by definition, therefore weighs 12 grams.
All other atomic masses of the elements are based on the carbon-12 standard. One unified atomic mass unit (u) is equal to 1/12 the mass of a carbon-12 atom — carbon-12 weighs 12 u. The mole is defined this way purely for convenience. Mathematically, it means that the mass of one mole of a compound in grams is equal to its molecular mass in u.
The molecular mass of a compound can be found by adding up the atomic masses of its constituent atoms. Water, for example, consists of two hydrogen atoms and one oxygen atom. Each hydrogen atom has an atomic mass of about 1.008 u and oxygen’s mass is about 15.999 u. Water’s molecular mass is therefore about 18.015 u, and the mass of one mole of water is 18.015 g.
The amount of a solute in a chemical solution is measured in moles. One mole of salt, also known as sodium chloride, has a mass of about 58.5 g. If 58.5 g or one mole of salt were dissolved in one kilogram of water, the resulting solution would have a molality of 1 m. Further, if two moles of sodium chloride were dissolved in .5 kilograms of water, the molality would be 4 m.
A similar and more widely used measure of concentration is molarity (M). Molarity, unlike molality, is based on volume: it is the number of moles of a solute in one liter of a solution. Since the volume of a solution can change, expanding or contracting with changes in temperature, molarity is not always a dependable measure of concentration. Molality, which is based on the unchanging quantity of mass, is often more useful in situations where the temperature of the solution is variable.