How to keep Molarity and Molality straight.

[amy73]

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Hi..
I keep getting the two confused.. Is there any mnemonic to keep it straight?
Bye
Amy

 

[Jonathan13180]

Hi..
I keep getting the two confused.. Is there any mnemonic to keep it straight?
Bye
Amy

No mnemonic. Just think of it this way, the standard is MOLARITY, its easier to work with, etc, and molality (n of solute/kg solvent) is the little red headed step child no one likes. Hope this helps.

 

[smc927]

If it helps, remember MOLARITY is what you usually deal with in chem lab. It is the # moles in a liter of solution.

molar = liter
molal = weight

Molar like a tooth which sits in your mouth surrounded by fluid (spit). Fluid = liter.
Molal as if you were to say it with a mouth full of brownies. Keep eating those brownies and you'll gain weight.

 

[Medikit]

Molarity sounds cool and is abbreviated in cool fashion (M) and deals with moles per liter which makes a lot of sense because you are comparing a unit of molecules dissolved in a volume of solution.


Molality sounds lame and is abbreviated in a lame fashion (m?) and deals with moles per kg which is really dumb because you are comparing a unit of molecules dissolved or incorporated in a mass of some stuff. But usually it's a solution, which is totally ******ed, especially if it's water because the density is around 1 which means you should have used molarity to begin with.


Molality will probably be on a Kaplan test, but it's rarely on the mcat.

 

[Phased]

M = mol solute / L solution
m = mol solute / kg solvent

Difference is molarity uses volume of solution, and molality uses mass of solvent.

 

[amy73]

Thank you Phased, I knew that.. I am looking for a mnemonic or an easy way to remember that..
Bye
Amy

 

[05med05]

1 M vs 1 m


1 M is slightly [M]ore concentrated than 1 m since the vlume of solute must be considered as part of the solution (slightly less solvent).

M = More concentrated, and how about this,
m = mass since we are dealing with mol/kg