Difference Between

a 1.0 Molar Solution vs a 1 Molal Solution

Long ago, it was hypothesized by Avogadro that there is a specific number that represents number of atoms or molecules of that substance in one mole of that substance. Thus one mole of each element contains equal number of atoms, irrespective of the atomic weight of that element. The concepts of molarity and molality were also developed to describe concentrations of a solute in a solution. While molarity is the measure of number of moles of the solute in a liter of solution, molality is the number of moles in 1kg of the solution. Armed with this knowledge, it is easy to find out the difference between a 1.0 Molar solution and a 1 Molal solution.

It is clear by the above definition that a 1.0 M solution contains 1 mole of solute per liter of solution. Remember, M is the symbol representing Molarity, while m is used to represent molality. Therefore, 1.0 m solution means 1 mole of solute per kg of the solvent. If water is the solvent, there is not much of a difference between 1.0 M solution and 1 m solution as at room temperature, the density of water is taken to be 1 kg/L. This results in molarity and molality of solutions being equal. However, for all other solvents, there is a difference in values of 1.0 M solution and 1.0 m solution. This is because all other solvents have a specific gravity more or less than one.

If one has to make 1.0 M solution of NaCl in water, one has to take 1 kg of water and add 23+35=58 g of NaCl (its atomic weight).

So if we were to take example of a solvent like ethanol whose density is 0.789kg/L, 1.0 M solution would be 0.789 m. Similarly, molality may be less or more than 1.0M solutions depending upon their density.