Molality. Calculation of Molality of Chemical Solutions

Molality is the ratio between the amount of matter in a solute (in moles - n₁) by the mass in kilograms of the solvent (m₂).

The symbol of molality is usually the W and it can be calculated by the following formula:

W = no₁
m₂

Molality can also be called concentration in quantity of matter per mass or molal concentration.

Mind Map - Mol

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See an example of how to perform this calculation:

“Calculate the molality of a solution that was prepared by dissolving 126.0 g of nitric acid (HNO₃) in 2.0 liters of water, whose density is equal to 1.0 g/mL. The molar mass of nitric acid is equal to 63.0 g/mol.”

First, we need to find the values ​​of n.₁ and of m₂:

no₁ = m₁
M₁
no₁ = 126.0 g
63.0 g/mol
no₁ = 2.0 mol

We can find out the value of the solvent mass (m₂) through the density formula:

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d = m → m = d. v → m₂ = (1.0 g/ml). (2000 mL) → m₂ = 2000 g or 2.0 kg of water
v
Now we can replace the values ​​in the molality formula:

W = no₁
m₂
W = 2.0 mol
2.0 kg
W = 1.0 mol/kg or 1.0 mole

In cases where there is 0.1 mol of solute or less dissolved in 1 liter of solution, the molality value will be practically equal to the molarity value (or concentration in mol/L).

Currently, this quantity is practically not used, but since its formula does not involve the value of the volume of the solution, which can vary with temperature, it is very useful to use when the temperature of the solution in question vary. It can also be useful in the study of colligative properties.

* Mind Map by Me. Diogo Lopes

By Jennifer Fogaça
Graduated in Chemistry

Would you like to reference this text in a school or academic work? Look:

FOGAÇA, Jennifer Rocha Vargas. "Molality"; Brazil School. Available in: https://brasilescola.uol.com.br/quimica/molalidade.htm. Accessed on June 28, 2021.