Molality of a solution is the ratio of number of moles of solute to the weight of solvent in kg. It is denoted by the symbol b or m. It is independent of temperature and pressure. Molality is different from molarity. Molality depends on mass of solvent where as molarity depends on volume of the solution.
Example 1: Find the molality of a solution prepared by dissolving 0.15 moles of with 555 gm of water at 75º C.
Here number of moles of = 0. 15
Mass of water or solvent = 555 gm
Here the temperature is 75º. But Molality does not depend on temperature.
1000gm is equal to 1 kg then 555gm is equal to 0.555 kg
Molality = = = 0.27 m.
Example 2: A sample contains 88gm of ethanol and 200gm of water at 26º C. Find the molality of the concentration.
Given, 88 gm of Ethanol
Number of moles of Ethanol = = 2.05 moles
Mass of water or solvent = 200gm.
Converting gm into kg. 1000gm is equal to 1 kg.
So 200gm is equal to 0.200 kg.
Molality = = = 10.25 m