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 a compound. This material is for higher tier only. In the last video we saw that a mole is simply a very large number, which I'm showing you here. We'll be looking more closely at this number in a later video.
We saw that if we're given a mass of an element, we can use this to calculate the number of moles of the element. We use this equation. The number of moles is the mass in grams divided by the relative atomic mass for that element. Now, in this video, we're going to learn how to calculate the number of moles of a compound.
In order to do this, we need to revise the idea of relative formula mass, which we saw in a previous video. This shows the compound carbon dioxide. We can see that we've got one atom of carbon.
bonded to two atoms of oxygen. We know that carbon atoms have a relative atomic mass of 12 and oxygen atoms have a relative atomic mass of 16. Adding these numbers together gives us the relative formula mass of carbon dioxide and we can see that this is 44. The symbol for relative formula mass is Mr and remember that relative formula mass has got no unit. So in this video we're looking at how to calculate the number of moles of a compound.
To do this we use this equation. The number of moles equals the mass that we're given in grams divided by the relative formula mass, Mr. As you can see this looks very similar to the last equation, except that now we're dividing by relative formula mass and not relative atomic mass. Let's look at an example. You're given a sample of calcium carbonate with a mass of 300 grams.
Calculate the number of moles of calcium carbonate in the sample. In order to calculate this, first we need to work out the relative formula mass for calcium carbonate. Calcium carbonate has the formula CaCO3. We've got one atom of calcium and calcium has a relative atomic mass of 40. We've got one atom of carbon and carbon has a relative atomic mass of 12. Finally we've got three atoms of oxygen and oxygen has a relative atomic mass of 16. Adding all of these together gives us a relative formula mass for calcium carbonate of 100. To calculate the number of moles of calcium carbonate in the sample, we need to divide the mass that we're given by the relative formula mass. So we've been given 300 grams of calcium carbonate and the relative formula mass is 100. This means that we have three moles of calcium carbonate.
Here's one for you to try. You're given 380 grams of magnesium chloride. How many moles of magnesium chloride have you been given?
The formula of magnesium chloride is MgCr2. The relative atomic mass of magnesium is 24. and the relative atomic mass of chlorine is 35.5. Pause the video now and try this yourself. OK, first we need to calculate the relative formula mass for magnesium chloride. So we've got one atom of magnesium, and magnesium has a relative atomic mass of 24. We've got two atoms of chlorine, and chlorine has a relative atomic mass of 35.5.
Adding all of these together gives us a relative formula mass for magnesium chloride of 95. To calculate the number of moles of magnesium chloride, we need to divide the mass that we're given by the relative formula mass. So we've been given 380 grams of magnesium chloride, and the relative formula mass is 95. This means that we've got 4 moles of magnesium chloride. Here's another one for you to try.
You're given a sample of lithium sulfate with a mass of 990 grams. Calculate the number of moles of lithium sulfate in the sample. The formula of lithium sulfate is Li2SO4.
The relative atomic mass of lithium is 7, the relative atomic mass of sulfur is 32, and the relative atomic mass of oxygen is 16. Pause the video now and try this yourself. Okay, so we start by calculating the relative formula mass of lithium sulfate. We've got two atoms of lithium and lithium has a relative atomic mass of 7. We have one atom of sulfur and sulfur has a relative atomic mass of 32. Finally we've got four atoms of oxygen, and oxygen has a relative atomic mass of 16. Adding all of these together gives us a relative formula mass for lithium sulfate of 110. To calculate the number of moles of lithium sulfate we need to divide the mass that we're given by the relative formula mass.
So we've been given 990 grams of lithium sulfate and the relative formula mass is 110. This means that we have nine moles of lithium sulfate in our sample. Okay here's one final example for you to try. You've been given 64.5 grams of beryllium hydroxide which has the formula BEOH2. Calculate the number of moles of beryllium hydroxide that you've been given.
I've given you the relative atomic masses, so you should pause the video now and try this for yourself. Okay, we'll start by calculating the relative formula mass for beryllium hydroxide. We've got one atom of beryllium, and beryllium has a relative atomic mass of nine.
We've got two atoms of oxygen, and oxygen has a relative atomic mass of sixteen. And finally we've got two atoms of hydrogen, and hydrogen has a relative atomic mass of one. Adding all of these together gives us a relative formula mass for beryllium hydroxide of 43. To calculate the number of moles of beryllium hydroxide, we need to divide the mass that we're given by the relative formula mass.
So we've been given 64.5 grams of beryllium hydroxide, and the relative formula mass is 43. This means that we have 1.5 moles of beryllium hydroxide. Remember that you'll find plenty more questions on calculating the number of moles of both elements and compounds in my revision workbook, which you can get by clicking on the link above. Okay, so hopefully now you should be able to calculate the number of moles of a compound.