Nobody buys lego just to let all the individual pieces sit there on the ground. The whole fun of it is putting pieces together to build large structures. Chemistry is the same way. Once you’ve learned the basics, you will eventually become interested in putting atoms together and making bigger, better more interesting structures called molecules. Heart medication, dynamite as well as the plastic on your PlayStation are all made possible by people who built new molecules using chemical reactions.
One of the first things you need to be able to do when starting your journey into chemical experiments is to keep track of how much — that is, how many grams and moles — of a substance you have.
MOLES TO GRAMS
For example, if I have 2 moles of hydrogen and one mole of oxygen, and then I let them react together to make one mole of H2O (water), I want to know how many grams I end up with. By using the atomic weight reading for each element, we can determine how many grams of water will be produced by this reaction:
So, as you can plainly see, one mole of water weighs 18.015 grams because it is made up of 2 moles of hydrogen and 1 mole of oxygen. All we had to do is add up the grams that 2 moles of hydrogen plus 1 mole of oxygen weigh and we got our answer.
Now, you may be wondering how we can add 2 moles of hydrogen and 1 mole of oxygen and end up with only 1 mole of water. After all, one plus two should be three, right? The reason is because each one water molecule is made up of three atoms: 2 hydrogen and 1 oxygen. Therefore, in order to have 6 water molecules, we would need 12 hydrogens and 6 oxygens. Likewise, in order to get 6.022 x 1023 molecules of H2O (a mole of water), we need 12.044 x 1023 (which is 6.022 x 1023 x 2) atoms of hydrogen and 6.022 x 1023 atoms of oxygen. So, although we altogether have 18.066 x 1023 atoms involved, they are snapped together into 6.022 x 1023 molecules made up of three snapped-together atoms. And, as we said, this single mole of water molecules weighs 18.015 grams.
GRAMS TO MOLES
Most of the time in chemistry, you will be given the number of grams of a molecule or element, and then asked to figure out how many moles you have. This is exactly like the question above except you work in the opposite direction: from grams to moles instead of moles to grams.
For example, if you are in a chemistry laboratory and are given 153.811 grams of carbon tetrachloride (CCl4), and then you are asked how many moles this gives you, how would you go about figuring it out?
Very easy. Just like in the above question regarding water, the first thing you need to establish is how many grams a mole of each element in the molecule weighs:
The result shows that one mole of carbon tetrachloride will weigh 153.811 grams. It just so happens that the amount of CCl4 we were given was exactly 153.811 grams. So we know that we have exactly one mole. Whenever you are given the number of grams of a substance you can easily determine how many moles you have by dividing the grams you have by the grams per mole of that substance. So, if we had been given 307.622 grams of CCl4, we would figure out the number of moles of CCl4 by the following formula:
Now you can know how to go from grams of a substance to the moles of that substance and vice versa.