 In this video we will be talking about how to calculate the masses of chemical compounds. Let's start with a little review. This is the mass of a proton and this is the mass of a neutron. These numbers are very awkward to use because of the exponents. Fortunately chemists use a different unit of measurement called the atomic mass unit. An atomic mass unit, or 1AMU, is approximately the mass of a proton or a neutron. The mass of electrons is so small that we can ignore it when we talk about the mass of an atom. If we want to know the atomic mass of a single atom, we add up the masses of the protons and the neutrons. For example, to calculate the approximate mass of a carbon atom, we add up 6 protons and 6 neutrons and get an atomic mass of 12 atomic mass units. You have probably noticed that the boxes in a periodic table have numbers on the bottom that have decimals rather than nice even numbers. And that's because these numbers represent average atomic masses. In nature, you don't usually just have one type of atom. You have several isotopes. Remember isotopes are atoms of the same element with different numbers of neutrons. Some isotopes are more common in nature than others. For example, a single chunk of carbon could have some carbon 12 in it with 6 protons and 6 neutrons. It might also have a little bit of carbon 13 with 6 protons and 7 neutrons and maybe a tiny, tiny amount of carbon 14, which has 6 protons and 8 neutrons. Carbon 12 is the most abundant isotope. The atomic mass reported in the periodic table is what we call a weighted average. And this means it accounts for the mass of each isotope as well as how much of each isotope there is. So because carbon 12 is most common in nature and because the other isotopes of carbon are present in very, very small amounts, the average atomic mass is 12.01 AMU, mostly carbon 12 with a little bit of the other isotopes. We use the term molecular mass or formula mass when we want to find the mass of a substance that's made from multiple atoms. In other words, when we want to find the mass of a compound. Which term we use depends on the type of compound we're talking about. For covalent compounds, we talk about molecular mass. For ionic compounds, we talk about formula mass. To calculate the molecular or formula mass, what we need to do is add up the average atomic masses of the atoms that make it up. When we do this, we need to make sure we multiply each average atomic mass by the number of copies of that atom that are in the compound. For example, in carbon dioxide with the chemical formula CO2, notice that there are two oxygen atoms. We need to account for both of these. When you want to determine a molecular mass or formula mass, sometimes it's helpful to make a table. Here's an example. What is the molecular mass of glucose with the chemical formula C6H12O6? And we will make a table to help us figure this one out. First thing I want to do is determine what the average atomic mass of carbon is. This is 12.01 atomic mass units. Now we need to find out how many carbon atoms are in the chemical formula and we see we have six. If we multiply the average atomic mass by the number of carbon atoms, we will get 72.06 AMU and that is the total mass of the compound that's made up of this element. Now let's look at hydrogen. From the periodic table, we see that hydrogen has an average atomic mass of 1.008 AMU. There are 12 of these in our chemical formula and if we multiply that by the average atomic mass, we get a total mass of 12.096 AMU from hydrogen. We'll do the same thing with oxygen. The average atomic mass is 16.00 AMU. We have six in our chemical formula and we multiply these together and get 96.00 AMU. We add up our total masses for all of our elements and our molecular mass of C6H1206 is 180.16 atomic mass units, rounded up to the nearest hundredths place. I think that when you're first doing this, it's easiest to start with a table, but you can also do it this way. We're doing the same thing, it's just in a little bit of a different format. Now let's do another example. What is the formula mass of CaCl2? That's calcium chloride. We say formula mass instead of molecular mass, since CaCl2 is an ionic compound and does not form molecules. Its actual structure looks more like this. Okay, let's make another table. We'll go to the periodic table and we'll look up our average atomic mass for calcium. We find its 40.08 atomic mass units. We have one of these in our chemical formula. We will multiply our average atomic mass by one and the total mass contributed by calcium is 40.08 AMU. Let's do the same for chlorine. The average atomic mass for chlorine is 35.45 AMU. We have two chlorine atoms in our chemical formula, so we will multiply the average atomic mass by two and get 70.90 atomic mass units. If we total up the masses from both elements, we get a total formula mass of 110.98 atomic mass units for calcium chloride. We could also set it up this way if we wanted to. For a more complicated example, what is the formula mass of iron III nitrate? It has this formula right here. We need to make sure when we count the atoms, we pay attention to the parentheses and the subscripts. So for more complicated examples like this, it may be helpful to count the atoms first. Looking at this formula, we have one iron atom, three nitrogen atoms, and nine oxygen atoms. Remember that subscript 3 outside the parentheses tells us that we have three copies of NO3. Now we'll make our table. We'll look up the average atomic mass of iron, and that's 55.85 atomic mass units. There is one of these in the chemical formula, and the total mass from this element is 55.85 atomic mass units. We'll do the same thing for nitrogen. The average atomic mass is 14.01 atomic mass units. There are three of them in our chemical formula, and the total mass from this element is 42.03 atomic mass units. Lastly, we'll go to oxygen. We have nine of these in our chemical formula. The total mass from oxygen is 144.00 atomic mass units. We are going to add all of these masses up, and the formula mass of iron III nitrate is 241.88 atomic mass units. In summary, the numbers you see at the bottom of the squares in the periodic table refer to the average atomic mass, which is a weighted average of the masses of the various isotopes in a sample found in nature. We use the term molecular mass or formula mass when we want to find the mass of a compound. To calculate molecular or formula mass, we add together the average atomic masses of the atoms that make up the compound. And when we do this, we pay attention to subscripts and the number of copies of each atom that the compound has.