Molarity is a chemical term that refers to the amount of a substance, usually a solute or solvent, that exists in a given volume of solution, and is more commonly known by terms such as molar mass or simply concentration. The reference is based on the basic molecular unit of the mol, mole, or gram molecule, which is loosely defined as the molecular weight of a chemical element or compound expressed in grams. Molecular weights vary from substance to substance as they are based upon the sum of of the weight of all of the atoms that bind together to make up basic molecules for the substance.
Calculating molarity in most instances in chemistry uses a base number of 12 as a reference point, with the isotope carbon-12 being the foundation for an atomic mass unit. A simple molarity formula as an example would be a combination of two hydrogen atoms that bind together in nature to form hydrogen-2, or deuterium, which has a molarity of two. Since the formula to calculate molarity must take into account the three dimensions present in a volume, molarity is expressed either as mols per cubic meter in standard international units, or as mols per liter, and a basic molar mass is defined as one mol per liter.
Molarity formulas become more complicated when necessary to determine the actual estimate for the number of molecules that exist in a particular volume or molar mass. These calculations are based on Avogadro's number, which is a very large number of 6.0225 x 1023, representing the number of molecules that exist as a mol of substance, and was initially based on the number of molecules in a gram molecule of oxygen. A more recent update to the idea is referred to as Avogadro's constant, which varies only slightly from the original number at 6.0221 x 1023 to accommodate changes in how standard international units are calculated as of 2011. Such fine levels of calculation for the quantity of molecules in a volume were first done by Lorenzo Avogadro, an Italian physicist and chemist of the 18th century, and related theories such as Avogardo's law, which determine the number of molecules in an ideal gas, were named after him.
The defined atomic weights of individual elements on the periodic table now make it possible to calculate the molarity of a given compound when the structure of each basic molecule is known. Information like this is useful in chemistry experiments such as those that involve acid molarity, or calculating the acidic and basic nature of solutions where either the molecular weight or the volume of the solution are known beforehand. This process is commonly referred to as titration, which involves adding reagents to a solution until it changes its acid or base nature, which can then be used to determine the molarity or quantity of molecules of the original constituent that was present.