Calculate The Molality of Each of The Following Solutions:
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Molality is a measure of solution concentration that expresses the number of moles of solute per kilogram of solvent. It's particularly useful in chemistry and biochemistry where the volume of the solution changes with temperature. This guide explains how to calculate molality and provides practical examples.
What is Molality?
Molality (m) is defined as the number of moles of solute dissolved in one kilogram of solvent. The key advantage of molality over molarity is that it doesn't change with temperature because it's based on mass rather than volume.
Molality is commonly used in:
- Chemical engineering
- Biochemistry
- Cryoscopy (freezing point depression)
- Ebullioscopy (boiling point elevation)
Molality is different from molarity (M), which is moles of solute per liter of solution. Molarity changes with temperature because volume changes, while molality remains constant.
Molality Formula
Molality (m) = moles of solute (n) / mass of solvent (kg)
The formula shows that molality depends on the amount of solute and the mass of the solvent, not the total volume of the solution.
Units
Molality is expressed in moles per kilogram (mol/kg).
How to Calculate Molality
- Determine the number of moles of solute (n)
- Measure the mass of the solvent in kilograms
- Divide the moles of solute by the mass of the solvent
- Report the result in mol/kg
Always ensure the solvent mass is in kilograms for accurate molality calculations.
Examples
Example 1: Sodium Chloride Solution
Calculate the molality of a solution made by dissolving 5.00 grams of NaCl in 250 grams of water.
- Moles of NaCl = mass / molar mass = 5.00 g / 58.44 g/mol ≈ 0.0856 mol
- Mass of water = 250 g = 0.250 kg
- Molality = 0.0856 mol / 0.250 kg ≈ 0.342 mol/kg
Example 2: Sugar Solution
What is the molality of a solution containing 20.0 grams of sucrose (C12H22O11) in 500 grams of water?
- Moles of sucrose = mass / molar mass = 20.0 g / 342.3 g/mol ≈ 0.0584 mol
- Mass of water = 500 g = 0.500 kg
- Molality = 0.0584 mol / 0.500 kg ≈ 0.117 mol/kg
| Solute | Mass of Solute (g) | Mass of Solvent (g) | Molality (mol/kg) |
|---|---|---|---|
| NaCl | 5.00 | 250 | 0.342 |
| Sucrose | 20.0 | 500 | 0.117 |
FAQ
What is the difference between molality and molarity?
Molality is based on mass of solvent (kg), while molarity is based on volume of solution (L). Molality doesn't change with temperature, whereas molarity does.
When should I use molality instead of molarity?
Use molality when the solution's volume changes with temperature, such as in cryoscopy or ebullioscopy experiments.
How does molality affect freezing point depression?
The freezing point depression (ΔTf) is directly proportional to molality: ΔTf = Kf × m × i, where Kf is the cryoscopic constant and i is the van't Hoff factor.