Finding Molality

General Chemistry Index

Where are we going with this? This page will assist in developing the ability to perform calculations to determine the composition of a compound or mixture when given the necessary information and to apply lab data to determine the empirical and molecular formula of a compound.

Finding Molality
Do we really need this? Some people do!

https://www.google.com/search?q=molality

Yet, again, we should whip up a definition!

Molality: the number of moles of something (the solute) in a kilogram of a solvent.

That… seems a lot like molarity. Indeed! But this is moles per kilogram, not moles per liter. And it is the mass of the solvent, not the mass of the solution.

Yet again, there should be a formula, right?

Where m is molality, mol is number of moles of solute, and kg is mass in kg of the solution, then…

m = mol/kg

Note: it is lowercase m for molality.

So, that doesn't look too hard… We should do an example!

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EXAMPLE:

Find the molality of a solution containing 5 moles of NaCl dissolved into 2 kg of water.

m = ?

mol = 5

kg = 2

m = mol/kg           <--- write the formula

m = 5 / 2            <--- plug in the numbers given, then do math

m = 2.5 mol/kg

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Well… that wasn't too bad!

Why is this even a thing?

https://www.google.com/search?q=where+is+molality+used

Working with concentrations when temperature is involved introduces changes in volume (because things contract and expand with decreases and increases in temperature). If you want to talk about the effect a concentration of something has, and don't want to discuss temperature, then you can use molality.