There are several ways to do this. The most accurate method for finding molecular mass is by mass spectometry. The volume of a given sample of a gas at room temperature is proportional to the number of moles of gas in the sample. With this in mind, we can work with volumes of gases as a way to set up conversion factors instead of always using moles. For reactions carried out in a solution, we express concentration in terms of molarity, M .
Molarity is the number of moles solute that are contined in one liter of solution.
To prepare a solution of a known concentration, one weighs out the required mass of solute and dissolve it into enough solvent to fill up to the line on a volumetric flask. One way of finding the concentration of a known acid or base is to perform an acid-base titration. A known volume of the acid or base whose concentration we don't know is placed in an erlenmeyer flask. A few drops of a pH indicator are added. In the balanced equation, formulate a conversion factor for moles of the buret substance to moles of the flask substance.