Molality Calculator

Calculate molality, moles, and solvent mass for colligative properties and solution chemistry

Calculation Mode

From Mass

Calculate molality from mass of solute

From Moles

Calculate molality from moles of solute

Find Mass Needed

Calculate mass to prepare solution

Molality ⇄ Molarity

Convert between molality and molarity

Mass of Solute

Molecular Weight (MW)

g/mol

Mass of Solvent

What is Molality?

Molality (m) is a measure of solution concentration defined as the number of moles of solute per kilogram of solvent. Unlike molarity, which depends on the volume of solution, molality is based on mass and therefore does not change with temperature. This makes molality particularly useful for studying colligative properties and temperature-dependent phenomena.

Molality Formula:

m = n_solute / mass_solvent

m = (mass_solute / MW) / mass_solvent

• m = Molality (mol/kg)
• n_solute = Moles of solute (mol)
• mass_solvent = Mass of solvent (kg)
• mass_solute = Mass of solute (g)
• MW = Molecular weight (g/mol)

Molality vs. Molarity

Property	Molality (m)	Molarity (M)
Definition	mol solute / kg solvent	mol solute / L solution
Units	mol/kg or m	mol/L or M
Temperature Dependence	Independent (mass-based)	Dependent (volume changes with T)
Best For	Colligative properties, high temperatures	Reactions, titrations, laboratory work
Ease of Preparation	Requires weighing solvent	Easier with volumetric glassware

Colligative Properties and Molality

Colligative properties depend on the concentration of solute particles, not their identity. Molality is the preferred concentration unit for these calculations because it's temperature-independent.

Freezing Point Depression

ΔT_f = K_f × m × i

• ΔT_f = Freezing point depression (°C)
• K_f = Cryoscopic constant (1.86 °C·kg/mol for water)
• m = Molality (mol/kg)
• i = van't Hoff factor (1 for non-electrolytes)

Example: 1 m solution lowers water's freezing point by 1.86°C

Boiling Point Elevation

ΔT_b = K_b × m × i

• ΔT_b = Boiling point elevation (°C)
• K_b = Ebullioscopic constant (0.512 °C·kg/mol for water)
• m = Molality (mol/kg)
• i = van't Hoff factor (1 for non-electrolytes)

Example: 1 m solution raises water's boiling point by 0.512°C

Osmotic Pressure

π = i × m × ρ × R × T

• π = Osmotic pressure (atm)
• ρ = Density of solution (kg/L)
• R = Gas constant (0.08206 L·atm/mol·K)
• T = Temperature (K)

Note: Often approximated using molarity for dilute solutions

Vapor Pressure Lowering

ΔP = X_solute × P°_solvent

• ΔP = Vapor pressure lowering
• X_solute = Mole fraction of solute
• P°_solvent = Vapor pressure of pure solvent

Raoult's Law: P_solution = X_solvent × P°_solvent

Worked Examples

Example 1: Calculate Molality

Problem: What is the molality of a solution prepared by dissolving 117 g of NaCl (MW = 58.44 g/mol) in 1000 g of water?

Step 1: Calculate moles of solute
n = mass / MW = 117 g / 58.44 g/mol = 2.002 mol

Step 2: Convert solvent mass to kg
mass_solvent = 1000 g × (1 kg / 1000 g) = 1.000 kg

Step 3: Calculate molality
m = n / mass_solvent = 2.002 mol / 1.000 kg = 2.002 m

Answer: 2.002 m (molal)

Example 2: Freezing Point Depression

Problem: What is the freezing point of a 2.0 m aqueous solution of glucose (non-electrolyte)?

Step 1: Use freezing point depression formula
ΔT_f = K_f × m × i

Step 2: Substitute values
ΔT_f = 1.86 °C·kg/mol × 2.0 mol/kg × 1 = 3.72 °C

Step 3: Calculate new freezing point
T_f = 0.00 °C - 3.72 °C = -3.72 °C

Answer: The solution freezes at -3.72 °C

Example 3: Mass of Solute Needed

Problem: How many grams of NaCl (MW = 58.44 g/mol) are needed to prepare a 1.5 m solution in 500 g of water?

Step 1: Convert solvent to kg
mass_solvent = 500 g × (1 kg / 1000 g) = 0.500 kg

Step 2: Calculate moles needed
n = m × mass_solvent = 1.5 mol/kg × 0.500 kg = 0.75 mol

Step 3: Calculate mass
mass = n × MW = 0.75 mol × 58.44 g/mol = 43.83 g

Answer: 43.83 g NaCl

Common Applications of Molality

Antifreeze Solutions

Calculating the concentration of ethylene glycol needed to prevent engine coolant from freezing at specific temperatures uses molality and freezing point depression.

Road Salt Calculations

Determining how much salt to use for de-icing roads based on desired freezing point depression of water/ice mixtures.

Molecular Weight Determination

Using freezing point depression or boiling point elevation to experimentally determine the molecular weight of unknown compounds.

High-Temperature Chemistry

For reactions or processes occurring at elevated temperatures where volume changes significantly, molality provides temperature-independent concentration values.

Biological Systems

Studying osmotic pressure and water movement in cells and tissues, especially in physiological temperature ranges.

Food Science

Calculating sugar concentrations in preserves and jams, or salt in brines, where boiling point elevation and preservation are important.

References

Molality calculations and colligative property formulas are based on fundamental physical chemistry principles:

Note: This calculator assumes ideal solution behavior. Real solutions, especially concentrated ones or those with strong solute-solvent interactions, may deviate from ideal behavior. Colligative property calculations use standard constants for water at 1 atm pressure. For electrolytes, remember to multiply by the van't Hoff factor (i) which accounts for ion dissociation.

Related Calculators

Molarity Calculator Normality Calculator Dilution Factor Calculator Percent Composition Calculator