Calculate The Molalities of The Following Aqueous Solutions:

Molality is a measure of the concentration of a solution expressed as the number of moles of solute per kilogram of solvent. This calculator helps you determine the molality of aqueous solutions quickly and accurately.

What is molality?

Molality (m) is defined as the number of moles of solute dissolved in one kilogram of solvent. Unlike molarity, which depends on temperature and volume, molality is temperature-independent because it's based on mass rather than volume.

Molality is particularly useful in applications where temperature changes might affect the volume of the solution, such as in freezing point depression calculations.

How to calculate molality

To calculate molality, you need to know:

The mass of the solute in grams
The molar mass of the solute in grams per mole
The mass of the solvent in kilograms

The calculation involves converting the mass of solute to moles and then dividing by the mass of the solvent in kilograms.

Molality formula

m = (mass of solute in grams / molar mass of solute in g/mol) / mass of solvent in kg

Where:

m = molality (mol/kg)
mass of solute = grams of solute
molar mass of solute = grams per mole
mass of solvent = kilograms of solvent

Worked example

Let's calculate the molality of a solution made by dissolving 25 grams of sodium chloride (NaCl) in 500 grams of water.

First, find the molar mass of NaCl. Sodium has an atomic mass of 23 and chlorine has an atomic mass of 35.5, so NaCl has a molar mass of 23 + 35.5 = 58.5 g/mol.
Convert the mass of NaCl to moles: 25 g / 58.5 g/mol ≈ 0.427 moles.
Convert the mass of water to kilograms: 500 g = 0.5 kg.
Calculate molality: 0.427 moles / 0.5 kg = 0.854 mol/kg.

The molality of this solution is 0.854 mol/kg.

FAQ

What is the difference between molality and molarity?

Molality is based on mass of solvent, making it temperature-independent, while molarity depends on volume, which changes with temperature. Molality is often preferred for freezing point depression calculations.

When should I use molality instead of molarity?

Use molality when temperature changes might affect the solution's volume, or when working with freezing point depression problems. For most other purposes, molarity is more commonly used.

How does molality affect freezing point depression?

The freezing point of a solution decreases with increasing molality. The relationship is described by the freezing point depression equation: ΔTf = Kf × m, where ΔTf is the freezing point depression, Kf is the cryoscopic constant, and m is the molality.