Molar concentration, commonly called molarity and often abbreviated M, is a measure of how many moles of a dissolved substance are present in a given volume of solution. It is widely used in chemistry and related sciences to express the composition of solutions because it links a chemical amount (moles) directly to a measurable volume. Overview of related concentration concepts can help when comparing different ways to quantify solutions.

Definition and units

By definition, the molar concentration of a constituent i is the amount of that constituent, n_i, divided by the volume of the solution, V: c_i = n_i / V. The amount n_i is measured in moles and volume V is usually expressed in liters, so the SI-derived unit for molarity is mol per litre (mol L−1), commonly called molar (symbol M). It is important to note that V denotes the total volume of the final solution, not the volume of solvent alone. Small changes in volume on mixing or with temperature therefore affect molarity. For further technical notes see concentration conventions.

Practical considerations and measurement

Preparing a solution of a desired molarity typically involves dissolving a known number of moles (or mass converted to moles) of solute in a volumetric container and then adding solvent to reach the final volume. Because volume varies with temperature, laboratories report molarity at a specified temperature when precision matters. Analytical methods used to determine molarity include volumetric titration, density measurements, and instrumental techniques tied to composition. Dilution follows the relation c1V1 = c2V2 when only solvent is added. The reciprocal of molarity appears in discussions of dilution and features in expressions such as Ostwald's law of dilution; see Ostwald for historical context.

The idea of relating amount of substance to volume arose as chemists developed the mole concept and standardized volumetric apparatus in the 19th and early 20th centuries. Terms like molarity helped make laboratory recipes and equilibrium calculations reproducible. Related concentration measures developed alongside molarity: molality (moles per kilogram solvent), mass fraction (mass percent), and normality (reactive equivalents per litre) each serve different practical needs. For background reading see historical notes and introductions in physical chemistry texts (basic, advanced).

Uses, examples and important distinctions

  • Titration: molarity is the usual unit used to express reagent concentrations for volumetric analysis.
  • Chemical equilibrium: equilibrium constants for reactions in solution are often expressed using molar concentrations.
  • Laboratory preparation: recipes for buffers, reagents, and culture media typically specify molar concentrations.
  • Industrial and environmental monitoring: reporting of contaminant levels or reactant feedstocks sometimes uses molarity for conversions to mass or flow quantities.

When choosing a concentration measure, consider the experimental variable most likely to change: use molality if temperature-induced volume changes are important, use mass percent for formulations defined by mass, and use molarity when volumes are convenient and reproducible. Additional comparative resources: molality vs molarity, mass fraction, normality, preparation guides, and analytical techniques.

Key points: molarity provides a direct and practical way to relate chemical amount to volume, but it depends on solution volume and temperature; other concentration scales may be preferable in some experimental or industrial contexts.