Molality Calculator
Understanding Molality
Chemists describe solution concentration using several related but distinct measures. While molarity refers to moles of solute per liter of final solution, molality captures moles of solute per kilogram of solvent. Because the denominator is a mass instead of a volume, molality stays constant even if temperature or pressure changes. This stability makes it the preferred concentration unit for experiments such as boiling point elevation and freezing point depression where the solution is deliberately heated or cooled.
To compute molality you first determine how many moles of solute are present. Divide the measured solute mass by its molar mass to obtain moles. Then divide by the solvent mass expressed in kilograms:
The result has units of mol/kg. Because typical lab balances report masses in grams, the calculator automatically handles the necessary conversions so you can focus on weighing the solute and solvent accurately.
Key Quantities
Each field in the form corresponds to a standard symbol used in thermodynamics and physical chemistry. The table below summarizes the notation and units so you can map your measurements to the formula.
| Quantity | Symbol | Typical unit |
|---|---|---|
| Mass of solute | g | |
| Molar mass of solute | g/mol | |
| Mass of solvent | kg | |
| Molality | mol/kg |
Example Scenarios
Molality and molarity are often similar for dilute aqueous solutions yet diverge for viscous or highly concentrated mixtures. The following comparison shows how the two measures evolve as solute loading increases.
| Solution scenario | Molarity (approx.) | Molality (approx.) |
|---|---|---|
| Very dilute saltwater | 0.10 M | 0.10 m |
| 1 kg water with 100 g NaCl | 1.7 M | 1.7 m |
| Boiling sugar syrup | 12 M | 15 m |
| Ethylene glycol antifreeze | 17 M | 18 m |
Practical Tips and Related Tools
When weighing reagents, tare the container before adding solute and solvent so you do not need to perform extra subtractions. Convert the solvent mass from grams to kilograms by dividing by 1,000, or let the calculator handle the conversion by entering the value directly. Keep notes on purity—impurities slightly reduce the effective molality.
The script validates each entry and only shows the copy button when a valid calculation is available. You can paste the summary into a lab notebook alongside results from the solution dilution, molarity, and boiling point elevation calculators to build a full picture of your solution plan. Because all math runs in your browser, none of the numbers leave your device.