Molarity vs Osmolarity
Concentration is an important phenomenon, and it is used very commonly in chemistry. It is used to indicate quantitative measurement of a substance. If you want to determine the amount of copper ions in a solution, it can be given as a concentration measurement. All most all the chemical calculations are using concentration measurements to draw conclusions about the mixture. To determine the concentration, we need to have a mixture of components. To calculate the concentration of each component’s concentration, the relative amounts dissolved in the solution have to be known. There are few types of methods to measure the concentration. They are mass concentration, number concentration, molar concentration, and volume concentration. All are ratios where the numerator is representing the amount of the solute, and the denominator is representing the amount of solvent. In all these methods, the way of giving the solute differs. However, the denominator is always the volume of the solvent.
Molarity is also known as molar concentration. This is the ratio between the number of moles of a substance in one volume of a solvent. Conventionally, the solvent volume is given in cubic meters. However, for our convenience we often use liters or cubic decimeters. Therefore, the unit of the molarity is mol per liter/ cubic decimeter (mol l-1, mol dm-3). The unit is also indicated as M. For example, a solution of 1 mol of sodium chloride dissolved in water has a molarity of 1 M. Molarity is the most commonly used method of concentration. For example, it is used in the calculation of pH, dissociation constants/ equilibrium constants, etc. Conversion of a mass of a given solute to its molar number has to be done in order to give the molar concentration. To do this, mass is divided by the molecular weight of the solute. For example, if you want to prepare 1 M of potassium sulfate solution, 174.26 g mol-1 (1 mol) of potassium sulfate should be dissolved in one liter of water.
In osmolarity, amount of solutes is given in osmoles. Only the solutes, which can dissociate within a solution, are given in osmoles. So, osmolarity can be defined as the number of osmoles (Osm) of a solute per litre (L) of solution. Therefore, the unit of osmolarity is Osm/L. Salts like sodium chloride are dissociated in solutions; therefore we can give an osmolarity value for them. For example, when a sodium chloride dissociates it gives a Na+ ion and a Cl– ion. So, when 1 mole of NaCl dissolves in water, it gives 2 osmoles of solute particles. When nonionic solutes dissolve, they do not dissociate. Therefore, they give only 1 osmole of solutes per 1 mole of solute.
What is the difference between Molarity and Osmolarity?
• Molarity means the number of moles of solute particles per unit volume of the solution, but osmolarity means the number of osmoles of solute particles per unit volume of the solution.
• Unit of molarity is mol dm-3 whereas unit of osmolarity is Osm/L.
• When a compound cannot dissociate when dissolved, osmolarity and molarity of that compound will be similar, but if the compound dissociates they will be different.