Molality Calculator
What This Calculator Does
This molality calculator determines the concentration of a solution by dividing the number of moles of solute by the mass of solvent in kilograms. Unlike molarity, molality (m) is temperature-independent because it's based on mass rather than volume.
Molality Formula
Primary Formula
m
Molality (mol/kg)
n
Moles (mol)
kg
Solvent Mass (kg)
n = m × kg
Calculate moles from molality
kg = n / m
Calculate solvent mass from moles and molality
Step-by-Step Example
Problem: Calculate the molality of a solution containing 3.0 moles of glucose dissolved in 2.5 kg of water.
- 1Identify the values: n = 3.0 mol, kg = 2.5 kg
- 2Apply the formula: m = n / kg = 3.0 / 2.5
- 3Calculate: m = 1.2 mol/kg
- 4Result: The solution has a molality of 1.2 m
Common Mistakes to Avoid
Using Solution Mass Instead of Solvent Mass
Molality uses only the solvent mass, not total solution mass
Confusing Molality with Molarity
Molality uses kg of solvent; molarity uses liters of solution
Not Converting Grams to Kilograms
Always divide grams by 1000 to get kilograms
Using Grams Instead of Moles
Convert mass to moles first using molar mass
Where Molality Is Used
Education
Taught in physical chemistry for colligative properties and thermodynamics
Laboratory
Colligative property experiments like boiling point elevation and freezing point depression
Industry
Temperature-sensitive applications where concentration must remain constant
Research
Physical chemistry research involving solution thermodynamics and phase equilibria
Related Calculators
Molarity Calculator
Calculate molar concentration in mol/L
Dilution Calculator
Calculate dilution using M₁V₁ = M₂V₂
Normality Calculator
Calculate equivalent concentration
Related Formulas
Frequently Asked Questions
What is molality in chemistry?
Molality (m) is a measure of concentration defined as moles of solute per kilogram of solvent. Unlike molarity, it's temperature-independent since it's based on mass, not volume.
How is molality different from molarity?
Molality uses kilograms of solvent (m = mol/kg) and doesn't change with temperature. Molarity uses liters of solution (M = mol/L) and varies with temperature as liquids expand/contract.
Why use molality instead of molarity?
Molality is preferred when temperature changes are involved (colligative properties, thermodynamics) because mass doesn't change with temperature, ensuring constant concentration values.
Can molality be calculated from molarity?
Yes, but you need solution density: m = (M × 1000) / (1000d - MM), where d is density (g/mL) and M is molar mass of solute. The conversion requires additional data.
What are colligative properties?
Colligative properties (boiling point elevation, freezing point depression, osmotic pressure, vapor pressure lowering) depend on the number of solute particles. Molality is used in their calculations.
How do you prepare a solution with specific molality?
Calculate required moles (n = m × kg), convert to grams using molar mass, weigh both solute and solvent separately, then dissolve the solute in the measured solvent mass.
Can molality be negative?
No, molality cannot be negative. Both moles of solute and mass of solvent must be positive values. A molality of zero means pure solvent with no solute.
What is a 1 m solution?
A 1 m (one molal) solution contains exactly 1 mole of solute dissolved in 1 kilogram of solvent. For example, 1 m glucose has 180.16g glucose per 1000g of water.