Hi and welcome back to freesciencelessons.co.uk. By the end of this video you should be able to calculate the number of moles of an element. This topic is for the higher tier only. Now I should point out that over the next few videos we're going to be looking at how we can use maths to carry out calculations and chemistry.
Some of the concepts can be a little tricky. I understand that some students may find these hard, so I'll do my best to explain them and I'll give you plenty of examples. However, what I would say is this, please don't give up. If you find something tricky, watch the video again and make sure that you look carefully at how I calculate the example questions. In the previous videos, we've been looking at the idea of relative atomic mass.
Relative atomic mass tells us the mass of a single atom of an element, and it's calculated from the average masses of the isotopes. weighted for their abundance. We find relative atomic masses on the periodic table, but you are given these in the exam. So for example, the relative atomic mass of carbon is 12, and the relative atomic mass of oxygen is 16. Remember that relative atomic mass has got no units.
In this video, we're looking at the idea of moles. Now, as I said, this can be a tricky idea. In chemistry, the word mole means a very large number.
I'm showing you this number here. We're going to look at this number in much more detail in a later video. Now scientists call this number one mole. It's a shorthand way of saying this number. The mole is a really useful concept in chemistry and we're going to see why now.
As we said before the relative atomic mass of carbon is 12. If I take 12 grams of carbon then I've got one mole of carbon atoms. In other words, this number of carbon atoms. Looking at oxygen again, we've seen that the relative atomic mass of oxygen is 16. If we take 16 grams of oxygen, then we've got one mole of oxygen atoms.
In other words, this number of oxygen atoms. This works for any element. If we take the relative atomic mass of any element in grams, then we've got one mole of atoms of that element.
In other words, this number. The mole is really useful because it allows scientists to calculate how many atoms of a substance they have in a chemical reaction. If we know the mass of an element that we're using in a chemical reaction, then we can calculate the number of moles using this equation. The number of moles equals the mass that we're given in grams divided by the relative atomic mass. It's really important that you learn this equation as we'll be using it a lot.
Here's an example. You're given a sample of magnesium with a mass of 48 grams. How many moles of magnesium have you been given?
If we look on the periodic table we can see that magnesium has a relative atomic mass of 24. To work out the number of moles we divide the mass of the element that we're given by the relative atomic mass. So in this case we're given 48 grams of magnesium and the relative atomic mass is 24. This means that we have two moles of magnesium. Here's one for you to try. You're given 120 grams of calcium. Calculate the number of moles of calcium you've been given and the relative atomic mass of calcium is 40. You should pause the video now and try this.
So we calculate the number of moles by dividing the mass we're given by the relative atomic mass. In this case we're given 120 grams of calcium and the relative atomic mass is 40. Putting these numbers into the equation tells us that we've got three moles of calcium. Here's another question. A sample of rock contains 252 grams of iron.
Calculate the number of moles of iron in the sample. The relative atomic mass of iron is 56. Again pause the video and try this yourself. Okay we've got 252 grams of iron and the relative atomic mass of iron is 56. Dividing the mass by the relative atomic mass tells us that we've got 4.5 moles of iron.
Here's one final example for you. You're given a sample of sulfur with a mass of 4064 grams. Calculate the number of moles of sulfur in the sample. The relative atomic mass of sulfur is 32. Pause the video and try this yourself.
So we've got 4064 grams of sulfur and the relative atomic mass of sulfur is 32. Dividing the mass by the relative atomic mass tells us that we've got 127 moles of sulfur in our sample. As you can see, mole calculations are not too tricky. You'll find plenty more examples of mole calculations in my revision workbook, which you can get by clicking on the link above. Okay, so hopefully now you can calculate the number of moles of an element.