Understanding Corrosion in Water Pipelines: A Guide for Pipeline Designers

# Molal Solution

Last updated: December 4, 2018

## What Does Molal Solution Mean?

A molal solution is a solution that contains 1 molecular weight of solute in a kilogram of solvent. It is the strength or concentration of a solution, especially the amount of dissolved substance in a given volume of solvent. It is a concentration of a solution expressed in moles or molality (m).

Because volume is not part of the molality equation, molality is independent of temperature. Using molalities rather than molarities for lab experiments keeps the results within a closer range.

In the field of electrochemistry and metal corrosion, molality and molarity are applied as concentration units.

## Corrosionpedia Explains Molal Solution

A solution obtained by dissolving one gram of the solute in 1000 grams of solvent is known as a 1 molal solution. For example, when 60 g of NaOH are dissolved in 1000 g of solvent, the solution contains 1.5 moles of solute in 1 kg of solvent. Therefore, the molality is 1.5.

The factors needed to calculate molality are moles of solute and the mass of solvent in kilograms. 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."

The primary advantage of using molality to specify concentration is that unlike its volume, the mass of the solvent does not change with changes of temperature or pressure; molality remains constant under changing environment conditions. Molality, like mole fraction, is used in applications dealing with certain physical properties of solutions.

As the molality changes, it affects the boiling point and freezing point (also known as the melting point) of the solution. A higher molality increases the boiling point and decreases the freezing point of the solution. As molality is a more accurate measure of solutes in solution in dynamic conditions, it is often used in comparing and determining colligative properties of solution.

Molality is a property of solutions. If the solvent is reactive, and one needs to know the stoichiometry between the solvent and the solute, knowing the molality can be very important. The mass-based nature of molality implies that it can be readily converted into a mass ratio.