 Okay, so if you are given a big balloon containing a mole of helium, there are a couple of things you can immediately work out. You would know that because of Avogadro's constant, there are 6.022 times 10 to the 23 atoms of helium in the balloon, and you could look up the periodic table and see that one mole of helium atoms weighs 4.003 grams, so the mass of the gas in the balloon is 4.003 grams. In this video, we're going to formalize these conversions so you can do them for any pure substance and any number of moles. And we're also going to add a new conversion that will let you work out how much volume gaseous substances occupy under certain conditions. These conversions are very like unit conversions, you're just changing the way you measure your substance. They're not mysterious or complicated, but sometimes it takes some practice to get smooth and quick at them, so do as much practice as you need until they become routine. They are the basis of most chemical calculations. So we're going to draw a map that will relate moles to the other ways of measuring substances. But let's leave our specific helium example here and make a more general map. So we'll start with moles. Remember this is our special number that essentially allows us to skip count how many atoms or molecules we have in a sample. Now in the same way that sometimes you need to convert dozens into a plain number, two dozen eggs is twenty-four eggs for example, sometimes we want to know the actual number of atoms or molecules in a sample rather than how many moles of atoms or molecules we have. So I'll put number of particles up the top here that something we'll want to convert to and from. Now what's the conversion factor that gets us between moles and numbers of particles? Well it's Avogadro's number, Na. 6.022 times 10 to the 23 tells us how many particles there are per mole. And the formula that relates these three quantities, Avogadro's constant, the number of particles you have and the number of moles you have, is here. Na equals particles divided by the number of moles. We'll do some examples using this in a few minutes. How else might we want to measure a sample? Well by mass. And the conversion factor that relates the moles and the mass of a particular substance is that substance's molar mass in grams per mole. Remember that this conversion factor has the same value as the mass of one mole but the units indicate that it's a ratio relating the number of moles to the mass. The formula is here. Molar mass equals mass divided by moles. Finally I'm going to introduce you to a third kind of conversion. This applies only to gases and we'll be exploring it more fully later in the course. For now you'll just have a taste. If you know the number of moles of a gas you can convert directly to its volume using the conversion factor called the molar volume. Before I show you how that works though I need to explain this little acronym here, STP. The volume of a gas changes if you change its temperature or pressure. So whenever you're stating a gas volume you have to say what the temperature and the pressure were when you measured that volume. They could be anything but scientists have defined a set of standard conditions which are useful when you're doing measurements or calculations. They're called standard temperature and pressure or STP. It's STP when the temperature is zero degrees Celsius or you could express it as 273 Kelvin. And the pressure is one atmosphere which is the average air pressure at sea level. Or if you measure it in kilopascals it's 101.3 kilopascals. So if we want to convert moles of gas into a volume of gas note that this really is just for gases it doesn't work for liquids. We use the molar volume. The molar volume is a ratio like molar mass. It says that a gas, any gas occupies 22.4 litres per mole. That is one mole of gas will occupy 22.4 litres. This is of course as long as you're at STP. If you change the temperature or pressure the molar volume will change and we'll learn how to deal with that later on. For now we're just going to assume that the gases are always at STP.