Molarity

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Molarity (M) is a term used in chemistry to indicate the concentration of a solute in a solvent, in units of M = moles/liter. For a pure compound with virtually no solvent, molarities are in the range of about 10-18 M. Chemical solutions typically used in labs are often between 10 millimolar (mM) and 1 molar concentrations. Solutions of biological samples, containing proteins, enzymes or DNA, are more often used in the nanomolar (10-9 M) or micromolar (10-6 M) range, up to about 10 mM. For a solution containing many molecules, each molecule will have a molarity which is independent of the other components, unless they chemically react over time. Molarity is calculated by dividing the amount (mass) of the chemical being added into a solution by both its molecular mass (molecular weight MW) and the final volume of the solution.


where

m = mass of chemical added, in grams
MW = molecular weight (mass), in grams/mole
V = final volume of the solution

Relation to normality

A related concentration unit, normality (N), is often more convenient when one is dealing with acid-base reactions, because a 1 N solution of any acid will always neutralize a 1 N solution of any base. Normality is a multiple of molarity.

where n is an integer.

For a monoprotic acid, 1M = 1N, while for diprotic and triprotic acids 1M = 2N and 1M = 3N, respectively.