# Half unit class 2 molar relationship between mass

### Stability of molar relationship after non-extraction Class II malocclusion treatment

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Molecular mass Molecular weight Video transcript - [Voiceover] What I wanna do in this video is get a little bit familiar with the idea of molecular mass. Let's start off with a molecule that we're all quite familiar with.

In fact, we are made up of this molecule or to a large degree, made up of this molecule. That is water and the molecular formula for water is H2O. Every water molecule has two hydrogen atoms and one oxygen atom.

What I'm curious about is, what is the mass of a molecule of water? So, the way that you might think about doing that is, what's the mass of each of the hydrogens? You're gonna have two of those and then what's the mass of the oxygen? That would be a reasonable way to do it. So, what is a mass of the hydrogen? A hydrogen has a mass We can go to a periodic table here, I've got one right over here.

You see hydrogen, it has its atomic number one. That's the number of protons, that's what makes it hydrogen. Then down here you have this number. I think you can see, it's a little bit small probably, on your screen, it says, "1. This right over here is the atomic weight of hydrogen. This is, if you take the various isotopes of hydrogen on the planet earth and you take the weighted average. So the ones that are more common, they are weighted higher.

The weighted average of their atomic masses is 1. Now for the sake of this first pass, we're just going to say, "It's roughly one atomic mass unit. This isn't precise and it's not how things are defined, but it is a useful way to think about atomic mass or mass on the atomic scale. Is that one neutron, or one proton have an atomic mass of approximately one atomic mass unit. In other videos we'll talk about why it's not defined this way and why it's based on the carbon isotope and all of that.

But this is a fairly useful thing that is helpful to think about, what is the atomic mass likely to be? What is it roughly? When you see that hydrogen, it has one proton. Clearly, that's what makes it hydrogen. It also has an atomic mass of one or roughly one, it says, "Well, it must not have a neutron. The electron also does contribute to mass but it's negligible compared to the proton. Most of this mass is essentially the proton.

A little bit of the electron and also kind of the weighted average, when you think about the other isotopes of hydrogen that maybe might have a neutron that is involved with it.

## Stoichiometry

For the sake of this, we could say, "Hey, hydrogen, it's a proton with an electron "kind of buzzing around it. Oxygen, same idea, we'll look at the periodic table.

You could guess what its mass is close to. Oxygen, the most typical isotope of oxygen that you'll see on earth has eight protons. The eight protons are what makes it oxygen. Well you look at the equation, you immediately see the mole ratio. So for every mole of this, so for every one atom we use of iron three oxide we need two aluminums.

So what we need to do is figure out how many moles of this molecule there are in 85 grams. And then we need to have twice as many moles of aluminum. Because for every mole of the iron three oxide, we have two moles of aluminum. And we're just looking at the coefficients, we're just looking at the numbers.

One molecule of iron three oxide combines with two molecule of aluminum to make this reaction happen. So lets first figure out how many moles 85 grams are. So what's the atomic mass or the mass number of this entire molecule? Let me do it down here. So we have two irons and three oxygens. So let me go down and figure out the atomic masses of iron and oxygen. So iron is right here, I think it's fair enough to round to Let's say we're dealing with the version of iron, the isotope of iron, that has 30 neutrons.

### “07” Half unit Class II molar relationship | idetulis.

So it has an atomic mass number of So iron has 56 atomic mass number. And then oxygen, we already know, is This mass is going to be 2 times 56 plus 3 times We can do that in our heads. But this isn't a math video, so I'll get the calculator out. That's 48 plusright, So one molecule of iron three oxide is going to be atomic mass units. So one mole or 6. So in our reaction we said we're starting off with 85 grams of iron oxide. How many moles is that?

Well 85 grams of iron three oxide is equal to 85 over moles. So that's equal to, 85 divided by equals 0. So everything we've done so far in this green and light blue, we figured out how many moles 85 grams of iron three oxide is. And we figured out it's 0. Because a full mole would have been grams.

But we only have So it's point 0. And we know from this balanced equation, that for every mole of iron three oxide we have, we need to have two moles of aluminum.

So if we have 0. So we're going to need 1. I just took 0. Because the ratio is 1: For every molecule of this, we need two molecules of that. So for every mole of this, we need two moles of this.

If we have 0. All right, so we just have to figure out how many grams is a mole of aluminum and then multiply that times 1. So aluminum, or aluminium as some of our friends across the pond might say. Aluminium, actually I enjoy that more. Aluminium has the atomic weight or the weighted average is But let's just say that the aluminium that we're dealing with has a mass of 27 atomic mass units.

So one aluminum is 27 atomic mass units. So one mole of aluminium is going to be 27 grams. So if we need 1.