Molality and Concentration of a Chemical Solution

The concentration of sucrose in water may be expressed in terms of molality.
The concentration of sucrose in water may be expressed in terms of molality. Uwe Hermann

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

Solution

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

Answer:

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.