Measurements of Mass (Molality) vs. Volume (Molarity)
Molality is an intensive property of solutions, and it is calculated as the moles of a solute divided by the kilograms of the solvent. Unlike molarity, which depends on the volume of the solution, molality depends only on the mass of the solvent. Since volume is subject to variation due to temperature and pressure, molarity also varies by temperature and pressure. In some cases, using weight is an advantage because mass does not vary with ambient conditions. For example, molality is used when working with a range of temperatures.
The molality, b (or m), of a solution is defined as the amount of substance of solute in moles, nsolute, divided by the mass in kg of the solvent,msolvent:
Molality is based on mass, so it can easily be converted into a mass ratio, denoted by w:
Compared to molar concentration or mass concentration, the preparation of a solution of a given molality is easy because it requires only a good scale; both solvent and solute are massed, rather than measured by volume. In many weak aqueous solutions, the molarity and molality are similar because one kilogram of water (the solvent) occupies one liter of volume at room temperature, and the small amount of solute has little effect on the volume of the solvent.
The SI unit for molality is mol/kg, or moles solute per kg of solvent. A solution with a molality of 1 mol/kg is often described as "1 molal" or "1 m." However, following the SI system of units, the National Institute of Standards and Technology, which is the United States' authority on measurement, considers the term "molal" and the unit symbol "m" to be obsolete, and suggests using mol/kg or another related SI unit instead.
It is easy to calculate molality if we know the mass of solute and solvent in a solution. Molality is an intensive property, and is therefore independent of the amount being measured. This is true for all homogeneous solution concentrations, regardless of if we examine a 1.0 L or 10.0 L sample of the same solution. The concentration, or molality, remains constant.
Calculating Molality Given Mass
If we mass 5.36 g of KCl and dissolve this solid in 56 mL of water, what is the molality of the solution? Remember that molality is moles of solute/kg per solvent. KCl is our solute, while water is our solvent. We will first need to calculate the amount of moles present in 5.36 g of KCl:
We also need to convert the the 56.0 mL of water to its equivalent mass in grams by using the known density of water (1.0 g/mL):
56.0 g of water is equivalent to 0.056 kg of water. With this information, we can divide the moles of solute by the kg of solvent to find the molality of the solution:
The molality of our KCl and water solution is 1.3 m. Since the solution is very dilute, the molality is almost identical to the molarity of the solution, which is 1.3 M.
Calculating Mass Given Molality
We can also use molality to find the amount of a substance in a solution. For example, how much acetic acid, in mL, is needed to make a 3.0 m solution containing 25.0 g of KCN?
First, we must convert the sample of KCN from grams to moles:
The moles of KCN can then be used to find the kg of acetic acid. We multiply the moles by the reciprocal of the given molality (3.0 moles/kg) so that our units appropriately cancel. The result is the desired mass of acetic acid that we need to make our 3 m solution:
Once we have the mass of acetic acid in kg, we convert from kg to grams: 0.12 kg is equal to 120 g. Next, we use the density of acetic acid (1.05 g/mL at 20 oC) to convert to the requested volume in mL. We must multiply by the reciprocal of the density to accomplish this:
Therefore, we require 114 mL of acetic acid to make a 3.0 m solution that contains 25.0 g of KCN.