Molarity, perhaps the concentration unit most often used by chemists, sometimes is just called concentration. In other words, the word concentration sometimes is used to imply 'molar concentration.' Molarity is defined as moles of solute per liter of solution. Because the volume of a solution rarely is the sum of the volume of the solute plus the volume of the solvent, the amount of solvent used when preparing a solution of know molarity is defined by the procedure used.

So, a one molar solution of sodium chloride contains one mole of sodium chloride per liter of solution.

One liter of sodium chloride solution would be prepared by weighing one mole of sodium chloride (58.4425 g NaCl), dissolving this in some water, say 600 mL of water, and then adding enough water to bring the final total volume of the homogeneous, well-mixed solution to precisely 1 liter. In this procedure, the total amount of water (mass or volume) that will be required remains unspecified except in this operational way.