Molality is a means of expressing the concentration of a chemical solution. Here's an example problem to show you how to determine it:
Sample Molality Problem
A 4 g sugar cube (Sucrose: C12H22O11) is dissolved in a 350 ml teacup of 80 °C water. What is the molality of the sugar solution?
Given: Density of water at 80° = 0.975 g/ml
Start with the definition of molality. Molality is the number of moles of solute per kilogram of solvent.
Step 1 - Determine number of moles of sucrose in 4 g.
Solute is 4 g of C12H22O11
C12H22O11 = (12)(12) + (1)(22) + (16)(11)
C12H22O11 = 144 + 22 + 176
C12H22O11 = 342 g/mol
divide this amount into the size of the sample
4 g /(342 g/mol) = 0.0117 mol
Step 2 - Determine mass of solvent in kg.
density = mass/volume
mass = density x volume
mass = 0.975 g/ml x 350 ml
mass = 341.25 g
mass = 0.341 kg
Step 3 - Determine molality of the sugar solution.
molality = molsolute / msolvent
molality = 0.0117 mol / 0.341 kg
molality = 0.034 mol/kg
The molality of the sugar solution is 0.034 mol/kg.
Note: For aqueous solutions of covalent compounds, such as sugar, the molality and molarity of a chemical solution are comparable. In this situation, the molarity of a 4 g sugar cube in 350 ml of water would be 0.033 M.