How to calculate moles using molality. Definitions, formula and different calculation methods explained, step by step, with practical examples.
Chemists use a variety of terms to describe the concentrations of various solutions . One of these is the mole , which is used to quantify the number of particles present in a substance.
Molality (simply denoted m ) is a measure of concentration that defines the moles of a solute present in one kilogram of solvent .
If you know the molecular weight of a chemical substance and the density of the mixture, it is not difficult to extrapolate the value corresponding to the number of moles. In this regard, in this guide we will see how to calculate moles using molality .
Calculate molecular weight
First, we need to calculate the product obtained by multiplying the molality by the molecular weight of the substance in grams. The term ” molecular weight ” means the mass in grams of one mole of a chemical substance. This measurement is easy to obtain: just consult the documentation that came with the purchase of the substance.
Let’s take sodium chloride (NaCl, or table salt) as an example , whose molecular weight is 58.44 g/mol. For a molal solution of 2, we calculate:
2 mol/kg x 58.44 g/mol = 116.88 g
The value obtained represents the grams of solute dissolved in one kilogram of solvent. If we add 1 kg, we obtain the total mass of the solution:
116.88g + 1,000g = 1,116.88g
Obtain the mass of the solution
Now you will need to perform another multiplication between the volume of the solution (expressed in milliliters) and the density of the solution (expressed in g/ml). From this product you will obtain the mass of the solution in grams.
To clarify this step, let’s consider an example : if you have 500 ml of solution with a density of 1.08 g/ml, the multiplication to be performed will be:
500ml x 1.08g/ml = 540g
Now set up an equation to see the correct proportion between the mass of 1 kg of solution and the real mass; let’s indicate with “X” the mass of the chemical contained in a given amount of solution. Using the example just given, you will have the following equation:
116.88 g NaCl / 1116.88 g solution = X g NaCl / 540 g solution.
Find the value of moles
Now all that remains to be done is to find the value of X. You can do this as follows:
X = (116.88 g NaCl / 1116.88 g) x 540 g = 56.5 g NaCl
Divide the value you just found (which corresponds to the mass) by the molecular weight of the chemical substance you are examining; in this way, you can convert the quantity into units of moles:
56.5 g / 58.44 g/mole = 0.967 moles of NaCl
However, this is not the only method to calculate moles using molality. In the next paragraph, we will analyze another procedure that also finds frequent application in chemistry. Let’s see what it is.
Calculate moles using a chemical reaction
There is another method to calculate moles using molality . Again, it is helpful to use an example to explain the process.
Suppose we have 60 grams of sodium hydroxide (NaOH) dissolved in 250 grams of water. To calculate the moles of the solute NaOH, we can use the following relationship:
n(NaOH) = m(NaOH) / M (NaOH)
Where:
- n is the number of moles,
- m is the mass in grams of the sample,
- M is the molar mass.
In the example in question, we have:
n(NaOH) = 60g / 40g/mol = 1.5 moles of NaOH
Since 250 grams equals 0.25 kg, we can calculate the molality as follows:
b(NaOH) = 1.5 mol / 0.25 kg = 6 mol/kg
So we calculated moles using molality.
