Calculate The Molality of The Solutions Having The Following Concentrations

Molality is a measure of solution concentration that expresses the number of moles of solute per kilogram of solvent. It's particularly useful in chemistry and biochemistry where the density of the solution changes significantly with temperature.

What is molality?

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

Molality formula

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

Molality is an intensive property, meaning it doesn't depend on the amount of solution present. It's particularly useful in cryoscopic and ebullioscopic measurements where temperature changes are used to determine solution properties.

Molality formula

The molality of a solution is calculated using the following formula:

Molality calculation

m = n / m_solvent

Where:

m = molality (mol/kg)
n = number of moles of solute
m_solvent = mass of solvent in kilograms

This formula is particularly useful when dealing with solutions where the volume changes significantly with temperature, as molality is independent of temperature changes.

How to calculate molality

To calculate molality, follow these steps:

Determine the number of moles of solute using the formula: n = mass of solute / molar mass of solute
Measure the mass of the solvent in kilograms
Divide the number of moles of solute by the mass of the solvent to get molality

Example calculation

For a solution with 5 grams of sodium chloride (NaCl) dissolved in 250 grams of water:

Molar mass of NaCl = 58.44 g/mol
Moles of NaCl = 5 g / 58.44 g/mol ≈ 0.0856 mol
Mass of water = 250 g = 0.25 kg
Molality = 0.0856 mol / 0.25 kg ≈ 0.342 mol/kg

Molality vs. molarity

While both molality and molarity measure solution concentration, they use different reference quantities:

Property	Molality	Molarity
Definition	Moles of solute per kg of solvent	Moles of solute per liter of solution
Symbol	m	M
Temperature dependence	Independent of temperature	Depends on temperature
Use in freezing point depression	Directly used	Not directly used

Molality is preferred in cryoscopic and ebullioscopic measurements, while molarity is more commonly used in other applications.

Practical applications

Molality has several important applications in chemistry and related fields:

Freezing point depression calculations
Boiling point elevation measurements
Osmotic pressure determination
Colligative property studies
Concentration measurements in biological systems

Understanding molality is essential for scientists working with solutions where temperature changes are significant factors.

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, while molarity changes with temperature.

When should I use molality instead of molarity?

Use molality when dealing with freezing point depression, boiling point elevation, or other colligative properties. Use molarity for most other concentration calculations.

How does temperature affect molality?

Molality is independent of temperature because it's based on the mass of the solvent, which doesn't change significantly with temperature.