The idea of molality refers to the number of moles of solute present in one kilogram of solvent.
To understand the concept of molality, it is essential to be clear about other notions from the field of chemistry . Especially those that are linked to solutions : homogeneous mixtures that are formed by dissolving a solid in a liquid. According to the International System of Units , molality is expressed as mol/kg .
A solution, therefore, consists of a solvent or solvent (the substance that allows it to dissolve) and a solute (the element that dissolves). The mixture is formed when the solute dissolves in the solvent.
What is molality
Returning to the idea of molality, this term refers to the number of moles of solute that are in each kilogram of solvent . It is, therefore, a measure of concentration.
It should be noted that a mole is a fundamental physical quantity that refers to a certain portion of material. Molality reflects the number of moles of the solute in one kilogram of the solvent. The formula is the following:
Molality = Moles of solute / Mass of solvent
m = n/kg
Molality, which is usually expressed in mol/kg , is independent of pressure and temperature . In this way your measurement can be carried out accurately.
Molality and molarity are similar concepts, although with differences.
Difference with molarity
Many times confusion occurs between molality and molarity since not only are the two terms very similar, but they also mention similar issues. Molarity indicates the amount of solute in one liter of solution : when working with volumes, it is associated with pressure and temperature conditions. If the pressure or temperature changes, the volume also changes and, consequently, the molarity changes. On the contrary, these types of alterations do not affect molality.
Having said all this, one of the advantages of molality over molarity is obvious, and it is precisely its "invulnerability", so to speak, against changes in pressure and temperature, the fact that its volume does not change. This makes it more precise when taking a measurement . Despite this, it is used less frequently than molarity.
Molality measurement
To measure molality in a laboratory, a beaker is used, and an analytical balance is used to determine the weight. It should be noted that before placing the concentration in the glass it is necessary to weigh it and then subtract this magnitude from the result.
The beaker is a cylindrical-shaped container made of borosilicate glass, a special type of glass that contains oxides of boron and silicon. Its thickness is thin and it is used very frequently in the laboratory , particularly for the preparation and heating of substances, as well as for the measurement and transfer of liquids.
Data to be considered
If we are asked to calculate the molality of a given solution, we also need to have three other pieces of information: the mass of the solvent, the amount of the solution, and its molecular weight . In the case of sulfuric acid ( H2SO4 ), for example, its molecular weight is 98 grams per mole. For a quantity of 80 grams and with a solvent mass of 400 grams, we should proceed as follows:
* First of all we must calculate the number of moles, which in the molality equation is represented by the variable n . To do this, we divide the mass by the molecular weight, which gives us a result of 0.82 moles, once we cancel the unit grams , present in both terms;
* Since we already had the mass of the solvent, all that remains is to apply the molality equation, dividing the number of moles by the mass, to obtain 2.05 mol/kg. Note that to avoid errors we must convert the mass data to kilograms, from 400 grams to 0.4 kilograms.