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Molality Calculator

Calculate the molality of a solution from solute and solvent masses. Search 130+ compounds for automatic molar mass lookup.

💡 Quick Summary

Calculate the molality of a solution from solute mass, molar mass, and solvent mass. Search 130+ common laboratory compounds for automatic molar mass lookup, or enter a custom value. Supports mg, g, and kg units for both solute and solvent.

📋 How to Use

Choose your input mode: Select Compound to search the built-in database, or Manually enter molar mass for a custom compound.
If using Select Compound, type in the search box — results filter instantly. Click or press Enter to select. The Molar Mass field auto-fills.
Enter the mass of the solute and select the appropriate unit (mg, g, or kg).
Enter the mass of the solvent and select the appropriate unit (mg, g, or kg).
Click Calculate to see the molality and step-by-step calculation.
Click Reset to clear all fields and start again.

🧮 Formulas & Logic

Molality (m)

m = n_solute ÷ m_solvent (kg)

Moles of Solute

n = mass_solute (g) ÷ M (g/mol)

Combined Formula

m = [mass_solute (g) ÷ M (g/mol)] ÷ mass_solvent (kg)

Unit of Molality

mol/kg — also written as "molal" or abbreviated as "m"

📊 Result Interpretation

mol/kg unit

Molality is expressed in mol/kg (molal). It is independent of temperature and pressure, unlike molarity.

Molality vs Molarity

Molality uses solvent mass, not solution volume. For dilute aqueous solutions, molality ≈ molarity. They diverge significantly at high concentrations or with dense solvents.

Colligative properties

Molality is the correct concentration unit for boiling point elevation (ΔT_b = K_b × m) and freezing point depression (ΔT_f = K_f × m).

Choosing solvent mass

Enter the mass of the pure solvent only — not the total solution mass (solvent + solute). This is the most common source of error.

🔬 Applications

Calculating boiling point elevation and freezing point depression (colligative properties)
Preparing precise standard solutions for analytical chemistry
Thermodynamic studies where temperature-independent concentration is required
Pharmaceutical formulation and quality control
Electrochemistry and battery electrolyte research
Food chemistry and flavour analysis
Cryoscopy — determining molar mass of unknown solutes

⚠️ Common Mistakes & Warnings

Solvent mass ≠ Solution mass

Molality is calculated using the mass of the solvent only, not the total solution mass. Entering the total solution mass (solvent + solute) will underestimate molality.

Check the correct compound form

Many compounds exist in hydrated and anhydrous forms with different molar masses (e.g., MgSO₄ vs MgSO₄·7H₂O). Select or enter the molar mass that matches the actual form you are using.

Unit mismatch

Double-check your selected units. A common mistake is entering a value in grams while the unit dropdown is set to kg (or vice versa), which gives a result 1000× too high or too low.

❓ Frequently Asked Questions

What is molality and how is it different from molarity?

Molality (m) is the number of moles of solute per kilogram of solvent. Molarity (M) is moles per litre of solution. Molality is preferred in thermodynamic calculations because, unlike molarity, it does not change with temperature (mass does not expand or contract with temperature, but volume does).

Why does molality use solvent mass and not solution volume?

Using solvent mass makes molality temperature-independent. Volume changes with temperature, so a molar solution prepared at 20°C has a slightly different concentration at 80°C. A molal solution maintains the same concentration across all temperatures.

How do I find the molar mass of my compound?

Use the compound search to find your substance — molar mass will be filled in automatically. For unlisted compounds, calculate it by summing the atomic masses of all atoms in the formula (e.g., NaCl = 22.99 + 35.45 = 58.44 g/mol).

Can molality and molarity be approximately equal?

Yes — for dilute aqueous solutions (< 0.1 mol/kg), molality ≈ molarity because the density of water is ~1 kg/L and the solute contributes negligible volume. At higher concentrations or with dense/light solvents, the two diverge significantly.

What is the n-factor shown for each compound?

The n-factor (equivalence factor) is used in normality calculations, not molality. It represents the number of reactive units per formula unit — for example, H₂SO₄ has an n-factor of 2 because it can donate 2 protons. It is listed here for reference and is used in other calculators.

How do I calculate freezing point depression using molality?

Use the formula: ΔT_f = K_f × m, where K_f is the cryoscopic constant of the solvent (1.86 °C·kg/mol for water) and m is the molality calculated here. For example, 1 molal NaCl in water depresses the freezing point by approximately 1.86°C (per ion pair).