Now that we know how to name both ionic and molecular compounds, it's time to learn how to calculate the mass of an ionic or molecular compound. Or its molecular mass. This is a type of stoichiometry. Remember that stoichiometry is the quantitative relationship between elements in a compound or between molecules and atoms in a chemical reaction. In this video, I'll be demonstrating how we can use the periodic table and the atomic masses that are on the periodic table, to calculate the mass of a compound. Looking back at the key tasks that we're learning how to do that involves stoichiometry, this is the second task in the list. In this, video we'll talk about calculating the mass of a molecule. This type of stoichiometry is extremely simple. A good analogy that we can use is using various coins to calculate a total amount of money. Here I've shown some United States currency. A quarter, a dime, a nickel and a penny. And each of those have certain valuations. The quarter is worth 25 cents, the dime is worth 10 cents, the nickel is worth five cents and the penny is worth one cent. Now when I'm in the United States I could quickly pull out change from my pocket and tell you how much money I have. But when I go to another country, I'm not as familiar with their currency, what their coins look like for example. And it would take me quite a bit longer to figure out how I could count out the amount of money that I needed to to pay for something. And this has normally resulted in me just holding out a handful of change to a vendor when I'm at a, in a foreign country and I have a lot of coins left at the end. They always find that to be pretty amusing. Let's suppose that I reach into my pocket and I pull out one quarter, two nickels, and two pennies. Well, how much money would that be? If you're from the United States or Canada, or somewhere who uses very similar types of currency, you can quickly see that it's 37 cents, but let's look closely at what type of calculation you're doing. What your really doing in your head is your saying the quarter is worth 25 cents the nickles are worth five cents each and I have two of those so now I'm up to 35 cents and then finally I add on the last two cents of the two pennies and I get a total of 37 cents. So I'm looking at the value of each item in the chemical formula here. Or here it's the monetary formula. And I'm adding those values together. We do the same thing when we calculate molecular masses. Let's go back to our friend nitrous acid, HONO. The formula is the key here. So the formula is HNO2 and looking on the periodic table remember we can look up the mass of each type atom. If we wanted to look up the mass of one molecule of nitrous acid we would need to add together 1 times 1, because each hydrogen atom weighs 1 atomic mass unit. 1 times 14, because each nitrogen weighs 14 atomic mass units, and 2 times 16, because there are two oxygen atoms and each of those weighs 16 atomic mass units. I don't expect you to have these values memorized, of course you'll look them up on the periodic table. Most periodic tables do have the atomic masses listed. Remember the atomic number is the smaller number that doesn't have the decimal places after it. Atomic mass is the larger number that does have the decimal places after it. So if we added those numbers together, the formula mass for nitrous acid is 47 atomic mass units. We just used the formula to calculate that. Now you try one. [BLANK_AUDIO] Very good. Since so many of you got that right, I'm just going to move along and try a more complicated type of module. Let's calculate the mass of one molecule of calcium nitrate. Calcium nitrate has one calcium, two nitrogens, remember, and six oxygens. So when we're performing the calculation to determine the formula mass, we need to use 1 times 40 for the calcium, 2 times 14 for the nitrogen and 6 times 16 atomic mass units for the oxygens. We add all those together and we get a formula mass of 164.1 amu. I forgot to take the am off there. That's an old way of writing atomic mass units, now it's just abbreviated with a u. I'm a creature of habit there. Alright, now that we know how to calculate the mass of one molecule, we can translate this into moles of molecules. Since one molecule of nitrous acid weighed 47 atomic mass units, one mole of nitrous acid must weigh 47.0 grams. And I use the word weight interchangeably with mass, I know they are not exactly the same, but I'm assuming we're always on the planet earth, so when I say weigh I mean take the mass of something. I know some of you are already getting ready to write a post about me misusing this word. I should be saying mass, but I'm going to say weigh here, because that's what we call it in the United States. You don't ask somebody, what's your mass, you say, how much do you weigh? Although that wouldn't be a very polite question in many social circles. Since we know how much one mole of nitrous acid weighs, it weighs 47 grams, remember, that's a lot of molecules. It's 6.02 times 10 to the 23rd molecules of nitrous acid has a mass of 47 grams. We can calculate how much smaller piles of nitrous acid weigh, or how much larger piles of nitrous acid weigh. Because we can turn this formula mass into a ratio. I can say, I can write this ratio two ways actually. This is kind of interesting to do. I can say 47 grams in 1 mole. Or I also can write the ratio upside down. I can say one mole weighs 47 grams. This is what's called a formula mass, this grams per mole way of writing it. Here's a couple of questions for you to try to calculate on your own. What is the mass of one half of a mole of nitrous acid? How many grams would that be? Well done, so when you set up your calculation, what you did was you said, okay I'm starting with one half mole of nitrous acid. I have a ratio that there's 47 grams per one mole. I can cancel the moles of nitrous acid, and I'm left with units of grams, and you calculated that there are 23.5 grams in a half a mole of nitrous acid molecules. We can do the same type of calculation to determine the weight or the mass of 3.2 moles of nitrous acid molecules. Go ahead and try that one now. Well done, we did exactly the same operation. We use the formula mass as a ratio, 47 grams per one mole. We started with 3.2 moles of nitrous acid. Cancel out our moles, and we see that sample of nitrous acid weighs 150.4 grams. You can probably imagine that you could weigh out a sample. Let's say I weigh out five grams of a sample. You could then determine how many moles of sample that is. Let's go ahead and try that. How many moles of nitrous acid molecules would be contained in five grams of nitrous acid?