Molarity
See also: Molar concentration
Molarity (in units of mol/L, molar, or M) or molar concentration denotes the number of moles of a given substance per liter of solution. A capital letter M is used to abbreviate the units of mol/L. For instance:
\frac{2.0 \text{ moles of dissolved particles}}{4.0 \text{ liters of liquid}} = \text{ solution of 0.5 mol/L}.
The actual formula for molarity is:
\frac{\text{ Moles of solute}}{\text{ Liters of solution}} = \text{ Molarity of solution}.
Such a solution may be described as "0.50 molar." It must be emphasized that a 0.5 molar solution contains 0.5 moles of solute in 1.0 liter of solution. This is not equivalent to 1.0 liter of solvent. A 0.5 mol/L solution will contain either slightly more or slightly less than 1 liter of solvent because the process of dissolution causes the volume of the liquid to increase or decrease.
Following the SI system of units, the National Institute of Standards and Technology, the United States authority on measurement, considers the term molarity and the unit symbol M to be obsolete, and suggests instead the amount-of-substance concentration (c) with units mol/m3 or other units used alongside the SI such as mol/L[1]. This recommendation has not been universally implemented in academia or chemistry research yet.
Preparation of a solution of known molarity involves adding an accurately weighed amount of solute to a volumetric flask, adding some solvent to dissolve it, then adding more solvent to fill to the volume mark.
When discussing the molarity of minute concentrations, such as in pharmacological research, molarity is expressed in units of millimolar (mmol/L, mM, 1 thousandth of a molar), micromolar (μmol/L, μM, 1 millionth of a molar) or nanomolar (nmol/L, nM, 1 billionth of a molar).
Although molarity is by far the most commonly used measure of concentration, particularly for dilute aqueous solutions, it does suffer from a number of disadvantages. Masses can be determined with great precision as balances are often very precise. Determining volume is often not as precise. In addition, due to a thermal expansion, the molarity of a solution changes with temperature without adding or removing any mass.[2] For non-dilute solutions another problem is that the molar volume of a substance is itself a function of concentration so that volume is not strictly additive.
[edit] Molality
Molality (mol/kg, molal, or m) denotes the number of moles of solute per kilogram of solvent (not solution). For instance: adding 1.0 mole of solute to 2.0 kilograms of solvent constitutes a solution with a molality of 0.50 mol/kg. Such a solution may be described as "0.50 molal". The term molal solution is used as a shorthand for a "one molal solution", i.e. a solution which contains one mole of the solute per 1000 grams of the solvent.
Following the SI system of units, the National Institute of Standards and Technology, the United States authority on measurement, considers the unit symbol m to be obsolete, and suggests instead the term 'molality of substance B' (mB) with units mol/kg or a related unit of the SI[3]. This recommendation has not been universally implemented in academia yet.
Note that molality is sometimes represented by the symbol (m), while molarity by the symbol (M). The two symbols are not meant to be confused, and should not be used as symbols for units. The SI unit for molality is mol/kg. (The unit m means meter.)
Like other mass-based measures, the determination of molality only requires a good scale, because the masses of both solvent and solute can be obtained by weighing, and molality is independent of the physical conditions like temperature and pressure, providing advantages over molarity.
In a dilute aqueous solution near room temperature and standard atmospheric pressure, the molarity and molality will be very similar in value. This is because 1 kg of water roughly corresponds to a volume of 1 L at these conditions, and because the solution is dilute, the addition of the solute makes a negligible impact on the volume of the solution.
However, in all other conditions, this is usually not the case.
