molarity

Molarity is a way to express the concentration of a solute in a solution. It is defined as the number of moles of solute per liter of solution and is denoted by the letter M. The unit is moles per liter (mol/L).

The molarity M is calculated from the amount of solute (in moles) and the total volume of the solution (in liters) using the formula M = n / V, where n is the number of moles of solute and V is the final volume of the solution in liters. The volume used in this calculation refers to the finished solution, not just the solvent. Because volume can change with temperature, molarity is temperature dependent, whereas molality (moles of solute per kilogram of solvent) is not.

Preparing solutions with a known molarity typically involves weighing the appropriate amount of solute and dissolving it in a solvent, then diluting to a precise final volume in a volumetric flask. For a desired concentration M2 in volume V2, the required amount of solute can be determined by n = M2 × V2. If starting from a stock solution with concentration M1, the relation M1V1 = M2V2 (dilution equation) allows calculation of how much stock solution to use.

Examples: To make 1.000 L of 0.500 M NaCl, one needs 0.500 moles of NaCl, which is about 0.500 × 58.44 g = 29.22 g, dissolved and diluted to a final volume of 1.000 L. To prepare 0.500 L of a 0.111 M glucose solution (molar mass ~180.16 g/mol), weigh 10.0 g of glucose, dissolve, and dilute to 0.500 L.

Molarity is widely used in laboratory work for solution preparation, titrations, and quantitative analyses.