Calculating Molality Example Problem 1

Molality is a measure of the concentration of a solute in a solution. It is used primarily when temperature is a concern. Molarity depends on the volume, but volume can change when temperature changes. Molality is based on the mass of solvent used to create the solution because mass does not change as the temperature changes.

This molality example problem shows the steps needed to calculate the molarity of a solution given the amount of solute and the mass of the solvent.

Problem

Calculate the molality of a solution prepared from 29.22 grams of NaCl in 2.00 kg of water.

Solution

Molarity is calculated using the formula:

where moles_SOLUTE is the number of moles of solute, in this case NaCl
and Kg_Solvent is the mass in kilograms of the solvent.

First, calculate the number of moles of NaCl.

Using a periodic table, the atomic masses are:
Na = 22.99 g/mol
Cl = 35.45 g/mol

Add them together to get the molecular mass of NaCl

molecular mass NaCl = 22.99 g/mol + 35.45 g/mol
molecular mass NaCl = 58.44 g/mol

Convert grams to moles of the NaCl

moles NaCl = 0.50 moles

Plug in this and the mass of the water into the molarity formula

m = 0.25 moles/kg
or
m = 0.25 molal

Answer

The molality of the NaCl solution is 0.25 molal.

As you can see, calculations for molality are straightforward. Remember to find the number of moles of solute and the mass of the solvent and the rest is simple. If you do not know the mass of your solvent, you often know the volume. Use the density of the solvent to find the mass you need.