Hi and welcome back to freesciencelessons.co.uk. By the end of this two-part video you should be able to use Avogadro's constant to calculate the number of molecules and atoms in a given sample. This work applies only to higher students, so if you're doing foundation you don't need to do this.
Now I'm going to be honest with you, some of this is pretty challenging. Please trust me that you can do this and don't give up. You might need to watch the video a couple of times.
We've been looking at moles and we've already seen that one mole is simply a shorthand way of saying this number. Scientists call this number Avogadro's constant. Normally we write this in standard form which is 6.02 times 10 to the power of 23. Before we start working with Avogadro's constant, we're going to look at how we can calculate the number of moles of atoms in a given number of moles of a molecule.
Here's a sample question. Calculate the number of moles of atoms in one mole of water molecules. And water has a formula H2O.
Looking at the formula, we can see that one molecule of water contains two atoms of hydrogen and one atom of oxygen. So that makes three atoms in total. So if one molecule of water contains three atoms, then one mole of water molecules must contain three moles of atoms. Here's one for you to try.
Calculate the number of moles of atoms in one mole of methane molecules. Pause the video and try this yourself. We can see that one molecule of methane contains one atom of carbon and four atoms of hydrogen. In other words, five atoms in total. Because one molecule of methane contains five atoms, We know that one mole of methane molecules must contain five moles of atoms.
Try this one. Calculate the number of moles of atoms in one mole of calcium hydroxide. Pause the video now and work this out.
Okay, we can see that one molecule of calcium hydroxide contains one atom of calcium, two atoms of oxygen and two atoms of hydrogen. So five atoms in total. So we know that if one molecule of calcium hydroxide contains five atoms, then one mole of calcium hydroxide must contain five moles of atoms.
Okay, so now that we can calculate the number of moles of atoms, we're ready to start working with Avogadro's constant. Take a look at this question. Calculate the number of atoms in one mole of hydrogen chloride. You'll notice that now we're asked for the number of atoms, not the number of moles of atoms. Remember that one mole of hydrogen chloride contains Avogadro's constant of molecules.
In other words 6.02 times 10 to the power of 23. The formula of hydrogen chloride is HCl. This means that one molecule of hydrogen chloride contains one atom of hydrogen and one atom of chlorine. So two atoms in total.
So what this means is that if we have 6.02 times 10 to the power of 23 molecules of hydrogen chloride. we must have two times that number of atoms. So in other words we must have 1.204 times 10 to the power of 24 atoms.
Here's one for you to try. Calculate the number of atoms in one mole of sodium oxide Na2O. Pause the video and try this yourself. Okay one mole of sodium oxide contains 6.02 times 10 to the power of 23 molecules of sodium oxide. In other words Avogadro's constant.
Looking at the formula of sodium oxide we can see that one molecule contains two atoms of sodium and one atom of oxygen. In other words three atoms in total. Therefore we need to multiply the number of molecules of sodium oxide by three to get the number of atoms in one mole of sodium oxide. This gives us a final answer of 1.806 times 10 to the power of 24 atoms. You'll find plenty more questions on using Avogadro's constant in my revision workbook, which you can get by clicking on the link above.
In the next video, we'll continue exploring how to use Avogadro's constant to answer more complicated questions.