0.5
mol/kg
1
kg
0.5
mol/kg
1
kg
The Molality Calculator computes the molal concentration of a solution from the moles of solute and the mass of solvent. Unlike molarity, molality (m) is defined as the number of moles of solute per kilogram of solvent, making it independent of temperature since mass does not change with temperature.
Enter the moles of solute and the mass of solvent in grams to calculate molality. This concentration unit is particularly important for colligative property calculations such as boiling point elevation, freezing point depression, and osmotic pressure.
Molality is calculated as:
m = moles of solute / mass of solvent (kg)
Note that the denominator uses the mass of the solvent alone, not the total solution. The mass in grams is converted to kilograms by dividing by 1000.
For example, dissolving 0.5 mol of NaCl in 1000 g (1 kg) of water gives a molality of 0.5 mol/kg. Molality is preferred over molarity in thermodynamic calculations because it does not change with temperature or pressure.
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Half a mole of NaCl dissolved in 1 kg of water gives a 0.5 molal solution.
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Dissolving 2.0 mol of sucrose in 500 g of water yields a 4.0 molal solution, which is quite concentrated.
Molality is independent of temperature because it is based on mass (which does not change with temperature) rather than volume (which expands or contracts). This makes molality essential for colligative property calculations and for precise work in physical chemistry where temperature varies. Molarity is more convenient for volumetric work in the laboratory.
Colligative properties depend only on the number of solute particles, not their identity. They include boiling point elevation, freezing point depression, vapor pressure lowering, and osmotic pressure. Molality is the preferred concentration unit for these calculations because it is temperature-independent and directly proportional to the mole fraction of solute.
To convert, you need the solution density (d) and the molecular weight of the solute (MW): m = M / (d - M x MW/1000). For dilute aqueous solutions, molarity and molality are approximately equal because the density is close to 1 kg/L and the solute mass is negligible compared to the solvent.
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