Molality
Moles of solute per kilogram of solvent.
Formula
Variables
Example
Used for boiling-point elevation
How it works
Molality measures how much solute is dissolved relative to the mass of the solvent, so it tells you the concentration of a solution without depending on temperature. The ratio works because moles count particles of solute, while kilograms of solvent measure how much medium is available to contain them. Using solvent mass instead of total solution volume makes molality especially useful when a solution’s volume changes with heating or cooling.
When to use it
Use molality when concentration must stay reliable across temperature changes, such as in colligative property calculations or laboratory preparation of solutions by mass. It is also useful when the volume of the solution is hard to measure accurately but the masses can be measured precisely.
Worked example, step by step
A chemist dissolves 0.75 mol of glucose in 250 g of water. Step 1: Convert solvent mass to kilograms: 250 g = 0.250 kg Step 2: Use the molality formula: b = n_solute / m_solvent Step 3: Substitute the values: b = 0.75 mol / 0.250 kg Step 4: Calculate: b = 3.0 mol/kg Final answer: The solution has a molality of 3.0 m.
Common mistakes to avoid
- Using liters of solution instead of kilograms of solvent
- Forgetting to convert grams of solvent to kilograms
- Using total solution mass instead of just the solvent mass
Did You Know?
Molality uses mass instead of volume, so unlike molarity it does not drift as temperature changes.
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