Molarity

Molarity (symbol: M) is the most common way chemists express the concentration of a solution. It is defined as the number of moles of solute dissolved per liter of total solution volume. A 1 M solution of sodium chloride, for example, contains exactly 1 mole of dissolved sodium chloride in every liter of solution.

Molarity matters because most quantitative lab work — from titrations to reaction stoichiometry — requires knowing exactly how much of a dissolved substance is present in a given volume of liquid. Preparing solutions of precise molarity, or diluting a stock solution to a target molarity, are everyday tasks in analytical and industrial chemistry.

Because molarity is a ratio of moles to volume, calculating it always starts by converting the mass of solute into moles using its molar mass. Tools like the Concentration Calculator and Dilution Factor Calculator handle both the mole conversion and the volume math automatically.

Molarity (M) = Moles of Solute / Volume of Solution (L)

For dilutions, the relationship between an initial concentrated solution and a diluted solution is:

M1 × V1 = M2 × V2

Where:

• M1, V1 = molarity and volume of the original (stock) solution
• M2, V2 = molarity and volume of the diluted solution

You dissolve 20 grams of sodium hydroxide (NaOH, molar mass 40 g/mol) in enough water to make 500 mL (0.5 L) of solution.

Moles of NaOH = 20 g / 40 g/mol = 0.5 mol

Molarity = 0.5 mol / 0.5 L = 1 M

Now suppose you want to dilute 100 mL of this 1 M solution down to 0.2 M. Using M1V1 = M2V2:

1 M × 100 mL = 0.2 M × V2, so V2 = 500 mL

You would need to add water to bring the total volume to 500 mL. Check your own numbers with the Dilution Factor Calculator.

• Molarity starts with moles: Every molarity calculation converts a mass of solute into moles first, since molarity is defined per mole, not per gram.
• Dilution follows a fixed ratio: Adding solvent lowers molarity proportionally without changing the amount of solute, which is exactly the relationship the M1V1 = M2V2 formula describes.
• pH is derived from molarity: The acidity or basicity of a solution is calculated directly from the molar concentration of hydrogen or hydroxide ions in it.
• Buffers rely on precise molarity: A buffer solution resists pH change only when its acid and conjugate base components are present at carefully controlled molar concentrations.
• Temperature affects molarity slightly: Because molarity is volume-based and liquids expand when heated, the molarity of a solution can shift marginally with temperature even though the moles of solute stay fixed.