Molality Calculator
Calculate concentration with precision. Temperature Independent solution analysis.
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Select a mode and enter values to find Molality, Moles, or Solvent Mass.
Understanding Molality
Molality (m) is the preferred concentration unit for thermodynamic calculations because it relies on Mass rather than Volume. This makes it immune to temperature fluctuations that cause liquids to expand or contract.
Molality Formula
Moles per Kilogram
Comparison
Key Applications
Molality is essential in these areas of chemistry:
- Boiling Point Elevation: Calculating how much a solute raises the boiling point of a solvent.
- Freezing Point Depression: Determining how antifreeze works to lower the freezing point of water.
- Vapor Pressure Lowering: Raoult's Law calculations.
Caution
Do not confuse "Mass of Solution" with "Mass of Solvent".
In Molarity, the volume refers to the total solution. In Molality, the mass refers ONLY to the solvent (e.g., just the water, not the salt + water).
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Frequently Asked Questions
What is Molality (m)?
Molality (m) is a measure of concentration defined as the number of moles of solute per kilogram of solvent. Unlike Molarity, it deals with mass, not volume. Unit: mol/kg or molal.
What is the formula for Molality?
The formula is: m = n / mass_solvent(kg), where 'n' is moles of solute.
Why is Molality temperature independent?
Volume changes with temperature (thermal expansion), which affects Molarity (mol/L). However, Mass does not change with temperature. Since Molality is based on mass (mol/kg), it remains constant regardless of temperature changes.
What is the difference between Molality and Molarity?
Molarity (M) = moles / Liter of Solution (Volume-based). Molality (m) = moles / Kilogram of Solvent (Mass-based). Molarity is easier to measure in labs (pipettes/flasks), but Molality is more precise for thermodynamics.
When should I use Molality instead of Molarity?
Use Molality when working with: 1) Significant temperature changes, 2) Colligative properties (Boiling Point Elevation/Freezing Point Depression), or 3) Very concentrated solutions where volume is non-additive.
How do I convert grams of solvent to kg?
Divide the mass in grams by 1000. For example, 500g of water = 0.5 kg.
What is a '1 Molal' solution?
A solution containing exactly 1 mole of solute dissolved in exactly 1 kilogram of solvent.
Does Molality equal Molarity for water?
For very dilute aqueous solutions at room temperature, they are nearly identical because the density of water is approx 1 kg/L. However, they diverge as concentration increases or if the solvent is not water.
How is Molality used in Freezing Point Depression?
The formula is ΔTf = Kf · m · i. To calculate the change in freezing point, you validly need the Molality (m), not Molarity.
Can I calculate Molality from Mass Percent?
Yes. Assume you have 100g of solution. The mass percent gives you mass of solute (convert to moles) and mass of solvent (100g - mass_solute). Then calculate m = moles / kg_solvent.