Chemistry: Calculating Molarity I

So, how do you calculate the molarity of a solution? What is molarity? Come to that, what is a mole? (And I don’t mean the small furry kind that leaves hills in the middle of your lawn…)

The Mole

A Mole – but not a chemistry mole!

A mole of something (in chemistry terms) is the amount of a substance that contains the same number of particles (atom, ions, molecules) as you would find in 12 g (grams) of carbon-12. Great, so how many atoms are there in 12 g of carbon-12? And what is carbon-12?

Well, carbon-12 is an isotope of the element carbon. About 99% of carbon is carbon-12 (there are also carbon-13 and carbon-14 atoms). So, carbon-12 is an isotope of an element, in fact it is the most abundant form (isotope) of carbon.

In 12 g of carbon-12 you would have (and this is a very BIG number), 6.0022 x 10²³ atoms. Or put another way: 600,220,000,000,000,000,000,000 atoms. This number is also called Avogadro’s number (or the Avogadro constant).

So, 1 mole of substance would contain 6.0022 x 10²³ atoms, and the molecular, or atomic, weight of a substance is the number of grams of that substance you would need to have 6.0022 x 10²³ atoms, ions, or molecules.

Some examples:

The atomic weight of sodium (Na) is 22.9899 g.mol^-1 (grams per mole)

So, 22.9899 g of sodium, which is 1 mole, would contain 6.0022 x 10²³ atoms.

The molecular weight of sodium chloride (table salt) is 58.44 g.mol^-1

So, 58.44 g of sodium chloride, which is 1 mole, would contain 6.0022 x 10²³ molecules. Hence, 58.44 g of sodium chloride would have the same number of molecules as atoms in 22.9899 g of sodium. (The fact that both have sodium in them does not matter.)

And finally, 11.49495 g of sodium is 0.5 moles as 11.49495 ÷ 22.9899 = 0.5, that is, it would contain 3.0011 x 10²³ atoms.

(The amount of a substance you have, divided by the molecular weight, equals the number of moles.)