Molarity

Molarity (M) expresses the moles of dissolved substance in each litre of solution. It is the relevant concentration unit whenever molecules interact with other molecules — receptor occupancy, binding affinity, enzyme kinetics — since these processes depend on the number of particles, not their mass. That is why in-vitro peptide publications quote nanomolar or micromolar values, whereas vial labels state milligrams.

Converting from mass to molarity

Moving from one to the other requires the molecular weight. For instance: 5 mg of a 1,419 Da peptide dissolved in 2 mL of bacteriostatic water produces 2.5 mg/mL; 2.5 g/L divided by 1,419 g/mol equals 1.76 mmol/L, i.e. 1.76 mM. A thousand-fold dilution of that stock brings it to 1.76 µM. Bear in mind that two peptides at identical mg/mL have different molarities whenever their masses are not the same — exactly why comparing an analog with its parent on a mass basis can be misleading.

Two caveats: molarity refers to a litre of solution, not of solvent, and it shifts with temperature because liquids expand. When high precision is needed, molality sidesteps both problems. Our molarity calculator and the moles and molarity guide take care of the arithmetic.

Related terms

micrograms vs milligrams · reconstitution · reconstitution calculator