Watch
Watching this resources will notify you when proposed changes or new versions are created so you can keep track of improvements that have been made.
Favorite
Favoriting this resource allows you to save it in the “My Resources” tab of your account. There, you can easily access this resource later when you’re ready to customize it or assign it to your students.
Molality
Molality is a property of a solution that indicates the moles of solute per kilogram of solvent.
Learning Objectives

Calculate the molality of a solution

Explain how molality is a colligative property
Key Points

Molality is a property of a solution and is defined as the number of moles of solute per kilogram of solvent.

The SI unit for molality is mol/kg. A solution with a molality of 3 mol/kg is often described as "3 molal" or "3 m." However, following the SI system of units, mol/kg or a related SI unit is now preferred.

Since the volume of a solution is dependent on ambient temperature and pressure, mass can be more relevant for measuring solutions. In these cases, molality (not molarity) is the appropriate measurement.
Terms

molality
The concentration of a substance in solution, expressed as the number of moles of solute per kilogram of solvent.

intensive property
A property of matter that does not depend on the amount of matter.

colligative property
A property of solutions that depends on the ratio of the number of solute particles to the number of solvent molecules in a solution, and not on the type of chemical species present.
Full Text
Measurements of Mass (Molality) vs. Volume (Molarity)
Molality is an intensive property of solutions, and it is calculated as the moles of a solute divided by the kilograms of the solvent. Unlike molarity, which depends on the volume of the solution, molality depends only on the mass of the solvent. Since volume is subject to variation due to temperature and pressure, molarity also varies by temperature and pressure. In some cases, using weight is an advantage because mass does not vary with ambient conditions. For example, molality is used when working with a range of temperatures.
Defining Molality
The molality, b (or m), of a solution is defined as the amount of substance of solute in moles, n_{solute}, divided by the mass in kg of the solvent,m_{solvent}:
Molality is based on mass, so it can easily be converted into a mass ratio, denoted by w:
Compared to molar concentration or mass concentration, the preparation of a solution of a given molality is easy because it requires only a good scale; both solvent and solute are massed, rather than measured by volume. In many weak aqueous solutions, the molarity and molality are similar because one kilogram of water (the solvent) occupies one liter of volume at room temperature, and the small amount of solute has little effect on the volume of the solvent.
Units
The SI unit for molality is mol/kg, or moles solute per kg of solvent. A solution with a molality of 1 mol/kg is often described as "1 molal" or "1 m." However, following the SI system of units, the National Institute of Standards and Technology, which is the United States' authority on measurement, considers the term "molal" and the unit symbol "m" to be obsolete, and suggests using mol/kg or another related SI unit instead.
Calculating Molality
It is easy to calculate molality if we know the mass of solute and solvent in a solution. Molality is an intensive property, and is therefore independent of the amount being measured. This is true for all homogeneous solution concentrations, regardless of if we examine a 1.0 L or 10.0 L sample of the same solution. The concentration, or molality, remains constant.
Calculating Molality Given Mass
If we mass 5.36 g of KCl and dissolve this solid in 56 mL of water, what is the molality of the solution? Remember that molality is moles of solute/kg per solvent. KCl is our solute, while water is our solvent. We will first need to calculate the amount of moles present in 5.36 g of KCl:
We also need to convert the the 56.0 mL of water to its equivalent mass in grams by using the known density of water (1.0 g/mL):
56.0 g of water is equivalent to 0.056 kg of water. With this information, we can divide the moles of solute by the kg of solvent to find the molality of the solution:
The molality of our KCl and water solution is 1.3 m. Since the solution is very dilute, the molality is almost identical to the molarity of the solution, which is 1.3 M.
Calculating Mass Given Molality
We can also use molality to find the amount of a substance in a solution. For example, how much acetic acid, in mL, is needed to make a 3.0 m solution containing 25.0 g of KCN?
First, we must convert the sample of KCN from grams to moles:
The moles of KCN can then be used to find the kg of acetic acid. We multiply the moles by the reciprocal of the given molality (3.0 moles/kg) so that our units appropriately cancel. The result is the desired mass of acetic acid that we need to make our 3 m solution:
Once we have the mass of acetic acid in kg, we convert from kg to grams: 0.12 kg is equal to 120 g. Next, we use the density of acetic acid (1.05 g/mL at 20 ^{o}C) to convert to the requested volume in mL. We must multiply by the reciprocal of the density to accomplish this:
Therefore, we require 114 mL of acetic acid to make a 3.0 m solution that contains 25.0 g of KCN.
Assign just this concept or entire chapters to your class for free.
Key Term Reference
 SI system
 Appears in this related concepts: Viscosity, Standard Units (SI Units), and Converting from One Unit to Another
 SI unit
 Appears in this related concepts: SI Unit Prefixes, SI Units of Pressure, and Bond Energy
 acid
 Appears in this related concepts: The Halogens (Group 17), Balancing Redox Equations, and Calculating Percent Dissociation
 aqueous
 Appears in this related concepts: Molecular, Ionic, and Complete Ionic Equations, Electrochemical Cell Notation, and Preparing a Buffer Solution with a Specific pH
 concentration
 Appears in this related concepts: Calculating Equilibrium Concentrations , Factors that Affect Reaction Rate, and Diffusion
 density
 Appears in this related concepts: Density Calculations, Volume and Density, and Quorum Sensing
 homogeneous solution
 Appears in this related concepts: Homogeneous versus Heterogeneous Solution Equilibria, Solvent Effects, and Kinetics
 liter
 Appears in this related concepts: Effect of a Common Ion on Solubility, Molar Solubility and Relative Solubility, and Secondary & Tertiary Structure of Large Peptides and Proteins
 molarity
 Appears in this related concepts: Solution Stoichiometry, Molarity, and Concept of Osmolality and Milliequivalent
 mole
 Appears in this related concepts: Equations of State, Avogadro's Number and the Mole, and Converting between Moles and Atoms
 ratio
 Appears in this related concepts: The Importance of Productivity, Basic Descriptive Statistics, and Schedules of Reinforcement
 solid
 Appears in this related concepts: Types of Synthetic Organic Polymers, Metagenomics, and Three States of Matter
 solute
 Appears in this related concepts: Solubility, Osmosis, and Electrochemical Gradient
 solution
 Appears in this related concepts: Electrolyte and Nonelectrolyte Solutions, Using Molarity in Calculations of Solutions, and Forming Your Thesis
 solvent
 Appears in this related concepts: The Common Ion Effect, Intermolecular Forces and Solutions, and Dilutions of Solutions
 substance
 Appears in this related concepts: DipoleDipole Force, Complex Ion Equilibria and Solubility, and Substances and Mixtures
 system
 Appears in this related concepts: Free Energy Changes for Nonstandard States, A Definition of Management, and Comparison of Enthalpy to Internal Energy
 temperature
 Appears in this related concepts: Extractive Metallurgy, Microstates and Entropy, and Temperature
 volume
 Appears in this related concepts: Volumes, Cylindrical Shells, and Shape and Volume
Sources
Boundless vets and curates highquality, openly licensed content from around the Internet. This particular resource used the following sources:
Cite This Source
Source: Boundless. “Molality.” Boundless Chemistry. Boundless, 16 Nov. 2014. Retrieved 21 May. 2015 from https://www.boundless.com/chemistry/textbooks/boundlesschemistrytextbook/solutions12/concentrationunits93/molality4015139/