Molality Formula

Experimental chemistry is a very subtle science and it puts a premium on accuracy of measurements. When setting up a chemical reaction, every one of the reactants needs to be mixed in the right proportions, to get a predetermined amount of product yield. Two of the most important chemistry formulas are those related to molarity and molality.

There are certain basic concepts in chemistry, which you must understand, in order to grasp more complex issues. One such basic concept, which is essential knowledge to understand molality, is the term - 'Mole of a compound.' It's quite easy to understand, if you know how to calculate molecular weight of any substance.

A mole of any substance or compound is its molecular weight expressed in the mass unit of 'grams'. The molecular weight of water is about 18 and, therefore, its molar mass is 18 gm. That is to say, 1 mole of distilled water will weight about 18 gm.

One more thing which you need to know about a mole of any compound is that it contains exactly 6.023 x 10²³ molecules. So, you know how many molecules of a compound are used in a reaction, when you know the number of moles used.

There are various ways in which, the concentration of a solution is measured. When you know the concentration, it is possible to gauge whether you are dealing with a saturated or unsaturated solution. One way of measuring concentration is by calculating molarity of the solute and the other one is through calculation of molality.

For any solute, the latter is defined as the number of moles, that are contained in every kilogram of the solvent. So, it measures the number of moles per kilogram of solvent used. If this solvent is distilled water and you know its volume used in liters, calculating its mass in kg is simple.

That is because, 1 liter of distilled water, exactly weighs 1 kg. So, in case a solution is prepared using distilled water, you can gauge the mass of the solvent, directly from its volume. Same is not true for other solvents. So, if you are using any other solvent, other than water, weigh it on a scale to know its mass in kilograms.

The definition provided above, must have given you a pretty good idea about what the formula will look like. Here it is: 

Molality of a Solute = [Number of Moles (N)/Mass of Solvent in Kg (M)] 

The unit used is Mol/kg. To calculate it, you need to know two parameters. One is the number of moles of the solute and the second one is the mass of the solvent in kilograms. 

Consider 117 gm of NaCl (Sodium Chloride or Common Salt) to be dissolved in 3 liters of distilled water. What will be the molality of NaCl in this solution? 

The molar mass of NaCl is 58.5 gm and therefore 117 gm of NaCl will have (117/58.5 = 2) moles of common salt. The mass of 3 liters of distilled water will be 3 kg. 

Molality of NaCl = N/M = 2 moles/3 Kg = 0.67 Mol/kg

Just ensure that you measure out the mass of the 'solvent', when calculating molality and not the mass of the entire solution. People often miss out on this minor detail, leading to an erroneous calculation.