How To Calculate Molecular Weight
Question
The small "u" represents a fifth of a Carbon-12 atom.
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How To Calculate Molecular Weight

Hi, my name is Charles and I am one of the math teachers from The Maxim Workshop. I'm just going to now teach you how to do some math. Hi! In this video, I am going to be showing you how to calculate molecular weight. Now, molecular weight is a quantity that is mainly used by physicists and chemists. So, for this example, what I'm going to do - I'm going to show you the molecular weight, or how to calculate the molecular weight of say, H20, which is a basic water molecule. Now, if we look at H2O, what we need to do first is to look at the molecular mass for each. Now, this is just basically a measurement of how big the nucleus is for each of these two elements. So, in your periodic table, you should find the molecular masses for each of these particular quantities, or elements I should say. So, Hydrogen - you should find, has a molecular weight of 1.00794u, and this 'u' is basically just a twelfth of say a Carbon-12 atom, and is just used as a standard kilogram measurement. And now, when we look at Oxygen, we see that Oxygen in the periodic table has a molecular mass of 15.9994u. Again, this 'u' is a twelfth of your Carbon-12 atom, which is basically used as a standard for kilogram measurements of atoms. Now, just to look at molecular weight, what we need to identify that it is not an actual weight measurement. It's more of a relative molecular mass reading - because gravity does not really play part on elements or molecules. It's just too small. So, what we want to do is find, basically, measurements that relate to a twelfth of a Carbon atom, and this 'u' does that for us. So the first thing we need to do is find out the molecular mass of a H2O, and then we are going to find out the relative molecular mass or molecular weight for an H2O molecule. So, what I'm going to do - I'm going to place these values now in a column so that we can actually just add them up. And we have, 15.99400. So, for this what I'm going to do - I'm just going to sum my 2 Hydrogen atoms with my Oxygen atom, given H2O to find the molecular mass. Okay, so we have here our two Hydrogen atoms, and our Oxygen atom, in terms of their 'u'-numbers or the molecular mass. And what we're going to do - we're going to sum these to find the overall molecular mass of our H2O. Now, we've got 8 here. We've got 9 plus 9 as 18, plus 4 as 22. We've got 9 plus 9 as 18 plus 4 which is 22, so put 2 there. We've got 9 plus 9 as 18, plus 7 which is 25 and we put the 2 there. We've got the 2 plus the 9 which is 11, 1 there, 1 there, 1 plus a 9 which is 8, and sorry, 10. Let's go back here. Okay, and off to that we're carrying the ones so we got 1 plus 9, which is just 10 - put the 0 there and the 1 here. And we've got 8, decimal point remember. And we have 1. So, basically the molecular mass of H2O is 18.01528u, okay? So now, all we have to do to find the molecular weight or as it's more commonly known, the relative molecular mass, is just divide or cancel off that 'u'. So, the molecular weight of H2O is just pretty much 18.01528 - and that's how to calculate molecular weight. .