Calculate The Molality for Each of The Following Aqueous Solutions

Molality is a measure of solution concentration that expresses the number of moles of solute per kilogram of solvent. This calculator helps you determine the molality for various aqueous solutions, which is particularly useful in chemistry, biochemistry, and pharmaceutical applications.

What is molality?

Molality (m) is defined as the number of moles of solute divided by the mass of the solvent in kilograms. The formula is:

Molality (m) = moles of solute / mass of solvent (kg)

Molality is particularly useful when dealing with solutions where the volume changes significantly with temperature, as it is independent of temperature changes. This makes it more reliable than molarity for certain applications.

How to calculate molality

To calculate molality, you need to know:

The number of moles of solute
The mass of the solvent in kilograms

The steps are:

Determine the mass of the solvent in kilograms
Calculate the number of moles of solute using its molar mass
Divide the moles of solute by the mass of the solvent in kilograms

Note: Molality is often used in freezing point depression and boiling point elevation calculations where temperature effects are significant.

Example calculations

Let's calculate the molality for a solution containing 5 grams of sodium chloride (NaCl) in 100 grams of water.

Calculate moles of NaCl: 5 g ÷ 58.44 g/mol (molar mass of NaCl) ≈ 0.0856 mol
Mass of water: 100 g = 0.1 kg
Molality = 0.0856 mol ÷ 0.1 kg = 0.856 m

This solution has a molality of approximately 0.856 m.

Molality calculation examples
Solute	Mass of solute (g)	Mass of solvent (g)	Molality (m)
Sucrose (C₁₂H₂₂O₁₁)	10	100	0.170
Potassium nitrate (KNO₃)	20	500	0.073
Calcium chloride (CaCl₂)	15	200	0.235

Comparison with molarity

Molality and molarity are both measures of concentration, but they differ in their definitions:

Molality: moles of solute per kilogram of solvent
Molarity: moles of solute per liter of solution

Molality is temperature-independent, while molarity changes with temperature because the volume of the solution changes. This makes molality more suitable for applications where temperature variations are significant.

Practical applications

Molality is particularly useful in the following scenarios:

Freezing point depression calculations
Boiling point elevation calculations
Osmotic pressure measurements
Vapor pressure lowering calculations

In these applications, molality provides a more accurate measure of concentration than molarity because it accounts for the mass of the solvent rather than the volume of the solution.

FAQ

What is the difference between molality and molarity?: Molality measures moles of solute per kilogram of solvent, while molarity measures moles of solute per liter of solution. Molality is temperature-independent, making it more suitable for certain applications.
When should I use molality instead of molarity?: Use molality when temperature changes are significant, as it remains constant regardless of temperature. Molarity should be used when the volume of the solution is constant and temperature changes are minimal.
How do I convert between molality and molarity?: To convert between molality and molarity, you need to know the density of the solution. The conversion formula is: Molarity (M) = Molality (m) × (Density of solution / Density of water).
What are the units for molality?: The units for molality are moles per kilogram (mol/kg).
Can molality be used for gas solutions?: Molality is typically used for liquid solutions. For gas solutions, other concentration measures like partial pressure or mole fraction are more appropriate.