What is the difference between molarity and molality

Molarity , also known as molar concentration, is the number of moles of a substance per liter of solution. Solutions labeled with the molar concentration are denoted with a capital M. Molality is the number of moles of solute per kilogram of solvent.

It is important the mass of solvent is used and not the mass of the solution. Solutions labeled with molal concentration are denoted with a lower case m. For aqueous solutions solutions where water is the solvent near room temperature, the difference between molar and molal solutions is negligible.

This means the "per L" of molarity is equal to the "per kg" of molality. For a solvent like ethanol where the density is 0. The important part of remembering the difference is:. Actively scan device characteristics for identification.

Use precise geolocation data. The amount of a given substance present in the given volume or the concentration of a given solution can be expressed in Molarity, Molality, Normality, etc.

The difference between Molarity and Molality is that molarity focus on the number of mole of solute per litres of the given solution, while molality deals with the mole of solute per kg of the solvent.

The molarity of a solution is dependent on the temperature and inversely changes with it. If the temperature is increased, the molarity decreases, while if the temperature is decreased, the molarity increases.

The value of molarity is affected by the change in pressure also. The value of molarity is not accurate or precise. The molality of a solution is independent of the change in temperature and pressure. This is because the mass of the solute remains unaffected by any change in temperature and pressure. To prepare a molar concentration one adds a known quantity of solute to a volumetric flask then fills up the flask with liquid until the 1 liter mark is achieved.

For example: one can make up a certain molar concentration of sugar. The weight of the sugar first has to be converted to moles and then the water is added until 1 liter is reached.

To calculate molarity you need the solute in moles, but usually you will have a certain weight of solute which means you first need to convert the grams into moles. This can be done by finding the molar mass of the solute from the periodic table. The molar concentration formula can be rearranged to solve for both volume and moles. Volume is influenced by changes in temperature or pressure. For instance, the volume would increase with increasing temperature. This means that there will be some question of accuracy where there are changes in temperature.

If temperature decreases enough then the liquid may contract causing the molarity to increase because the same number of moles remains but there would be less solution. Conversely, if the temperature increases enough then the liquid may expand causing the molarity to decrease because the same number of moles remains but there would be more solution present. Molarity can be used to calculate the concentration of a substance that has been diluted. Molarity can be used when exact precision is not required.

It is however influenced by changes in temperature because it is a volumetric measurement, so in some cases it may not be appropriate to use. Molality can be defined as the number of moles of substance known as the solute that is found in a certain mass of solvent given in kg, that it is dissolved in. An example of making a molal concentration would be to weight out a certain amount of sugar for instance.

This then needs to be converted into the number of moles using the molecular mass of the sugar. The solution includes both the solute and the solvent. Molality, on the other hand, is the ratio of the moles of a solute to the kilograms of a solvent. Note that the mass used in the denominator is that of only the solvent, not the solute and the solvent. Find a handy calculator for determining the molarity of a solution here.

A calculator for molality can be found here. This article was updated May 4, to clarify the definition of molality. Molality definition Molality m , or molal concentration, is the amount of a substance dissolved in a certain mass of solvent.