Monday, February 11, 2013

Chemistry - What is a mole?!

What is a mole?

A mole is just a count of how many items are present.

A decent comparison is to the ton. A ton of feathers weighs the same as a ton of bricks. The ton applies to weight, however. Mole is counting how many feathers or how many bricks, and not their weight.

The mole is a quantity. Avogadro's number is equal to 1 mole. So, if you have one mole of something, you have 6.022 x 10^23 individual items.

Do you have 6.022 x 10^23 footballs? You have one mole of footballs.

Do you have 6.022 x 10^23 straws? You have one mole of straws.

Does your neighbor, the crazy cat lady, have 6.022 x 10^23 cats? She has one mole of cats.

In chemistry, we don't count footballs, straws, or cats, however. We count molecules, atoms, or parts of atoms.

If you have a mole of molecules, you have 6.022 x 10^23 molecules. If you have 6.022 x 10^23 atoms, you have a mole of atoms.

This is useful for determining the compositions of unknown compounds, taken from measurements, or if you want to find out how much of a compound you need to make a solution of a particular concentration.

Avogadro determined that if you have hydrogen, which has a formula mass of 1.007, at the atomic scale, and you take 1.007 g of hydrogen, you have one mole of hydrogen. This applies to all of the elements, and even to molecules.

So for a molecule, you add up the formula masses of all atoms present, then take that amount in grams of that molecule (compound). Voila! You have one mole!