How To Calculate Mass

Hi, I'm Donald Sinclair, I'm a science teacher with Greater London Tutors, and today we're going to be looking at a few topics in chemistry. This is how to calculate mass in chemical equations. A mole is a unit that tells you how much of something there is.

It's defined as six times ten to the twenty-three atoms or ions or molecules or whatever's most appropriate. Six times ten to the twenty-three of course is six followed by twenty-three zeros, so it's a very large number, but because atoms, ions, and molecules are very small, it's appropriate to use a very large number. This is called Avogadro's number after the scientist who first came up with it.

This means that if you have one mole of water for example, this means that you have six times ten to the twenty-three water molecules, and so on. Now the relative formula mass of an atom or a molecule tells you how much one mole of that thing will weigh in grams, so for example, if you had one mole of magnesium, which means if you had six times ten to the twenty-three magnesium atoms, that means it would weigh twenty four grams. You can work out the formula mass for molecules very easily.

So for example, water, one molecule contains two hydrogen atoms, one oxygen atom which is one plus one plus sixteen. So the relative formula mass of water is eighteen. This means that one mole of water will weigh eighteen grams, or will have a mass of eighteen grams.

Let's look at an example where the idea of moles allows to predict what masses are involved in a chemical reaction. For example, here we have the reaction of magnesium with oxygen to form magnesium oxide. Now, it's already been balanced and the numbers tell you how much or how many moles of that substance will react with how many moles of the other.

So for example, we can see that two moles of magnesium react with one mole of oxygen molecules to form two moles of magnesium oxide molecules. Now, if we find the masses involved here, two moles of magnesium will have a mass of forty-eight grams, since one mole of magnesium has a mass of 24 grams. One mole of oxygen molecules will have a mass of sixteen times two, equals thirty-two grams.

One mole of magnesium oxide would be twenty-four plus sixteen equals forty. However, because there are two of them, we have a total mass right inside of eighty grams. Now, always the masses on the left-hand side should equal the masses on the right hand side.

So here forty-eight plus thirty-two equals eighty, which is what we have on the right hand side. So let's say we were asked to work out if we had three grams of magnesium reacting with XS oxygen, in other words, as much oxygen as is needed for the reaction. What mass of magnesium oxide would we have at the end? We work out in this way.

We know that one mole of magnesium has a mass of twenty-four grams. So if we only have three grams what fraction of a mole is that? Well, it would be three over twenty-four, which is point one two five, or an eighth of a mole. So we know that we have one eighth, or point one two five moles of magnesium reacting.

We can see from the equation that two moles of magnesium would give us two moles of magnesium oxide. In other words, it's a two to two ratio, or more simply a one to one ratio. That means if we have point one two five moles of magnesium reacting that means we're going to have point one two five moles of magnesium at the end.

So we also have point one two five moles of magnesium oxide. How much does, or what is the mass of one mole of magnesium oxide. We worked out earlier, it's twenty-four plus sixteen, it's forty grams.

So one mole has a mass of forty grams. However, we've just seen that we only have point one two five moles, so point one two five, which is the number of moles, times the mass of one mole which is forty, is point one two five times forty, which is five. So we have five grams of magnesium oxide, from three grams of magnesium reacting with oxygen. .