In this video I like to cover the topics formula weight and molecular weight. They both have to do with the weight of a compound. The formula weight, or sometimes called formula mass, is the weight of a single unit of a compound or a single molecule. The molecular weight, or commonly called the molar mass, is the weight of a large collection of molecules or atoms. But the weight numbers for both of these formula weight and molecular weight are the same. So these numbers are numerically the same, but they represent different physical amounts. And these weight numbers can be gathered from the periodic table. These numbers are the decimal numbers on the periodic table, not the whole numbers. For example, with iron, iron weighs 55.845. Cobalt weighs 58.933. These whole numbers, 26.27, mean something else which we'll cover in a different video. But I want you to focus on these decimal numbers. In this video, these are the weight numbers. First I'll cover formula weight. Formula weight is the weight of a single molecule or formula unit and is reported in the units of atomic mass unit or AMU. So think of AMU as a code for the actual weight of an atom or molecule. Because the weight of one AMU or one atomic mass unit is 1.67 times 10 to the minus 24 grams, which is extraordinarily small. To give you an appreciation of how light one AMU is, consider an egg. Now of course an egg to us is not very heavy. Well an egg, the average egg, weighs about 30 grams as shown in this picture. Now compare that to the weight of single molecules of these particular compounds. H2O, MGCO3, FE3PO4, etc. The formula weights and AMU were calculated using the decimal or weight numbers from the periodic table. For H2O or water, there are two hydrogens and one oxygen. Looking at a periodic table, hydrogen weighs 1.0079. I'll approximate that just to 1.0. So there's two of those. And to that we add the weight of oxygen, which is 15.9999 or I'll just round that up to 16. So 16 and 2 ones make 18. What I'd like for you to do is verify these other formula weights that are presented in this table. Here I showed how I determined 132 for the last one, NH42, SO4. The two outside of the parentheses mean that there's two of the NH4 parts. So there's two hydrogens and eight hydrogens. Then there's 1S and 4 oxygen. And nitrogen weighs 14. We know hydrogen weighs 1. So for 32 and oxygen 16. And it's okay to round these numbers to three significant figures or if you want two decimal places to the right. I included this third column to show you the actual weight and grams of a single molecule or formula unit. To do that, multiply the weight of a single AMU, which is 1.67 times 10 to the minus 24 grams, by the AMU weight of a single molecule or unit. You can see a single water molecule is approximately 3 times 10 to the minus 23 grams, etc. These are extremely, extremely small numbers. There's 22 zeros in front of that three. So down here, below the calculation of the AMU, I just showed the setup of how to determine the actual gram amount of a single molecule or formula unit. Multiply the AMU number by 1.67 times 10 to the minus 24 grams per AMU. And if you think of dimensional analysis, which is this is the way it's set up in that model, the unit of AMU cancels and you're left with the unit of grams. Now I'll cover molecular weight or molar mass. Molecular weight represents the weight of 1 mole or 6.02 times 10 to the 23rd formula units of a particular compound. And the molecular weight numerically is the same as the formula weight. The key difference is that the molecular weight is interpreted as 18 grams in the case of water per 1 mole of water. So if there are 6.02 times 10 to the 23rd molecules of water, that amount of water will weigh 18 grams. Think about that in the context of what 1 AMU weighs, which is an extremely small number, and what an egg would weigh. And so the molar masses for the other compounds in the table are the same as the numbers we've calculated before. Here is a picture showing 1 mole of a few elements. Sulphur, magnesium, can, silicon, and copper. These measured weights of these elements are very close to the weights listed in the periodic table. Here is another picture showing some weights of compounds. Aspen or C9H804, copper II chloride dihydrate, and iron III oxide, and water. Verify for yourself the molar masses or molecular weights of these compounds. And your calculation should produce a number very close to these weights presented to you in this picture.