Themes > Science > Chemistry > General Chemistry > Solution and Solubility > Solubility of Solutes and Aqueous Solutions > Solution Index > Molality

An alternative unit of concentration to molarity is molality. The molality of a solute is the number of moles of that solute divided by the weight of the solvent in kilograms. For water solutions, 1 kg of water has a volume close to that of 1 liter, so molality and molarity are similar in dilute aqueous solutions.

To figure out the molarity of a solution, simply work out the number of moles of the solute (Probably from the molecular weight) and divide by the weight of the solvent. It's probably the case that you're given a volume of solvent rather than the weight: use the density to convert between the two

molality (M) = moles solute/kg of solution
To convert a volume and a molality of a solution to moles of solute, simply solve the above equation for moles of solute:
moles solute = molality * kg of solution

Example 1: If you have 10.0 grams of Br2 and dissolve it in 1.00 L of cyclohexane, what is the molality of the solution? The density of cyclohexane is 0.779 kg/l at room temperature.

Solution 1: First, work out the number of moles of bromine. Br2 has a molecular weight of 159.8 g/mole, so we have

10 g / (159.8 g/mole) = 0.063 moles BR2
Next, convert the volume of solvent to the weight of solvent using the density
1.00 L * 0.779 kg/l = 0.779 kg
Now just divide the two to get the molality
0.063 moles Br2/ 0.779 kg cyclohexane = 0.080 molal


Information provided by: http://learn.chem.vt.edu