Title: Moles and Molarity
Course: BTEC-1100-301-302-303-F21 > Section: Module 1 - Calculations and Measurements

Another major way we describe concentrations is with molarity. To understand molarity, we’ll first have to understand moles. In laboratory science, a mole is not a small burrowing mammal, but instead it is a word that means a number. Just as “dozen” means 12, and “score” (as in “Four score and seven years ago…”) means 20, “mole” means 6.02 x 10^23.

Remembering the number 6.02 x 10^23 is not important for your work here, but it is important that you realize that a mole is simply another unit used to describe an amount. It tells you not the mass of molecules (like g), but how many molecules there are. The abbreviation for mole is “mol”. Always write out mol, rather than shortening to m, as m is already used for other things!

Bottles of laboratory chemicals list either the MW (molecular weight) or FW (formula weight). (The FW may be different from the MW, depending on how a chemical was made. Always check the bottle of the chemical you are actually using to ensure you have the correct MW or FW; Google does not know which form of a chemical is on your chemical shelf, and may provide the wrong value!) In chemistry, you’ll learn that the MW is the mass, in atomic mass units (amu), of a single molecule. (MW = amu/molecule) One atomic mass unit is the approximately the mass of one proton -- it is an incredibly tiny amount, and not something that can be measured in a typical biotech lab.

The “magic” (actually it’s science!) of the number 6.02 x 10^23 is that it allows us to define MW in a way that’s more practical in the lab. While MW does = amu/molecule, MW also = grams/mole of that molecule. In other words, the MW or FW tells you the mass in grams of one mole of a substance. The units of MW or FW are rarely provided on the bottle, so this is one thing you need to memorize.

Units for MW or FW are g/mol

Molarity (abbreviated with a capital M) is a measure of concentration. A 1 molar solution contains one mole of solute per liter of solution.

Molarity = M = mol/liter