In chemistry, molar concentration is defined as the concentration of a material (in moles) divided by the volume of the mixture. It is also called molarity, amount-of-substance concentration, amount concentration, substance concentration, or simply concentration. The volume in the definition refers to the volume of the solution, not the volume of the solvent. One litre of a solution usually contains either slightly more or slightly less than 1 litre of solvent because the process of dissolution causes volume of liquid to increase or decrease. The SI unit for molarity is is mol/m3 however, more commonly the unit mol/L is used. A solution of concentration 1 mol/L is also denoted as "1 molar" (1 M). 1 mol/L = 1 mol/dm3 = 1 mol dm−3 = 1 M = 1000 mol/m3. The symbol c is used to denote concentration in moles per liter, as is the chemical symbol in brackets [H], although such brackets are sometimes used to represent concentration in any units. It is important to distinguish moles from molarity; molarity is a measurement of concentration while moles are a measure of the amount of substance present at a given time (Figure 1).
For example, the expression cNaCl would be read as "the concentration of sodium chloride in moles per liter of solution." This is not the solution that would result from adding 1 mole of NaCl to a liter of water, since the total volume after mixing would be a little more than 1 liter. Sodium and chloride ions take up room, even when dissolved in water. The proper procedure in making a 1.0M solution would be to dissolve the salt in less than a liter of water, and then slowly add more water while mixing, until the total volume reached 1.00 liter.
The advantage of expressing concentrations in terms of molarity is that these solutions can now be used in chemical reactions of known stoichiometry. Knowing the molarity of a solution enables calculating the number of moles and the mass of solute present in the solution. Multiplying the molarity by the volume gives the amount of moles present; dividing the molarity by the amount of moles gives the solution volume. Molarity can be used to dilute more concentrated solutions to less concentrated solutions.
For example, the molar mass of potassium bromide is 119.0 g/mole. If 119.0 grams of KBr is dissolved in 1.000 L of water, the concentration would be 1.000 mole/L, or 1.000 M. If we now took half of this solution (0.500 L) we know that we would also have 0.500 moles of KBr.