Tutorials

# What is Molality? Definition and Formula

## Core Concepts

In this tutorial on molality, you will learn the definition, importance, and how to calculate molality in understanding the concentration of a solution. Example calculations are provided.

## Vocabulary

• Molality (m): A measurement of concentration using the number of moles of solute per mass of solvent
• Solutions: homogeneous mixtures comprising of a solute and solvent
• Colligative properties: characteristics of a solution that are dependent on the concentration of solute ions
• Molarity (M): A measurement of concentration using the number of moles of solute per liters of solution

## Solution Basics

Solutions are mixtures with evenly mixed, or homogeneous, components. Furthermore, these mixtures can be made of any state of matter. Solutions consists of two components: the solute and solvent. The solute is the minor or dissolved part of the solution. In comparison, the solvent dissolves the solute. For example, in the solution of table in water, the salt is the solute and the water is the solvent.

## What is Molality?

Molality, or molal concentration, refers to the number of moles of solute divided by the mass of solvent, in kilograms, mol∕kg. This term is useful in understanding the concentration of a solution because its formula is independent of temperature and pressure. Thus, the colligative properties are dependent on molality. The colligative properties include boiling point elevation, freezing point depression, vapor pressure lowering, and osmotic pressure.

## Calculating Molality

### Formula: m = mol of solute/ kg of solvent

Example: What is the molality of a 52 g sample of table salt (NaCl) in a 75 kg sample of water?

In this example problem, the mass of the solute (table salt) and mass of the solvent (water) are provided. Using the units of the molality formula (mol/kg), determine if the components of the example problem have the correct units. In this example, the number of moles of solute are not provided, but with the known mass of the solute, the number of moles of solute can be determined. To calculate the number of moles of NaCl, use the molar mass of NaCl. In 1 mole of NaCl, there are 58.44 grams of NaCl. With this molar mass of NaCl known, the conversion from grams to moles can be calculated via dimensional analysis.

Looking back at the molality formula, the correct units are mol/kg. The mass of the solvent with correct units is provided in the problem, so dimensional analysis is not needed. Thus, divide the number of moles of solute by the mass of the solvent to calculate the molality of the solution.

## Molality and Molarity

Similarly, molarity is useful in calculating the concentration of a solution. Molarity measures the number of moles of solute per volume of solution, mol∕L, or the number of moles per one liter of solution. Additionally, molarity is dependent on temperature and is impacted by the colligative properties.

## Molality Example Problems

Formula: m = mol of solute/kg of solvent

Remember to acknowledge units (mol/kg)

1. Calculate the molality of a solution consisting of 35 g MgCl2 dissolved in 100 mL of water. The molar mass of MgCl2 is 95.21 g/mol and the density of water is 1 g/mL.
2. Calculate the molality of a solution consisting of 20 g ethanol dissolved in 300 mL of acetone. The molar mass of ethanol is 46.07 g/mol and the density of acetone is 0.790 g/mL.
3. Calculate the number of grams of NaCl dissolved in 10 kg of water. The molality of the solution is 0.64 mol/kg and the molar mass of NaCl is 55.48g/mol.