 In this video we're going to continue with mole conversions but we'll make them a little more complex. We've looked at one step calculations, just going between moles and some other measurement. Here we'll look at converting from one of the green boxes on this map to another green box via moles. This uses exactly the same skills you developed previously, but we're just going to join two conversions together. For instance, if we need to convert between the mass of something and the number of particles in it, we would first convert from mass to moles and then from moles to particles. If we needed to convert from the volume of a gas at STP to its mass, we would first convert from volume to moles and then from moles to mass. So let's try this problem. What is the mass of 8.43 times 10 to the 24 molecules of ammonia? There are a couple of things we need to do before we can do this calculation. First of all, what's the formula of ammonia? It's NH3. Second, let's figure out a strategy for this problem. We're going from number of molecules to mass. So the way we're going to set this out is we will first do a conversion of molecules to moles and then we'll convert moles to mass. Alright, what conversion factors do we need for each of these conversions? Well, from molecules to moles, we need Avogadro's number. And from moles to mass, we need the molar mass of the substance, which in this case is ammonia. Now we know Avogadro's number, that's 6.02 times 10 to the 23, but I don't know the molar mass of ammonia off by heart, so I'm going to have to calculate that. So the molar mass of ammonia is going to be equal to the molar mass of nitrogen plus three times the molar mass of hydrogen because each ammonia molecule is made up of one nitrogen and three hydrogens. So I check the periodic table for the molar masses and I plug that into the calculator and that equals 17.031 grams per mole. Okay, I now have both of my conversion factors so I can go ahead and do this calculation. First of all, I write down what I know. That's the number of molecules of ammonia. Next, I write down the conversion factor that's going to get me from molecules to moles. It's 6.02 times 10 to the 23 molecules per mole. Now I can cancel out the molecules and I have moles. Now I could do this intermediate calculation and then start a new line, but there's actually no need to. I can continue on this same line and do the second conversion as part of the same calculation. I already know that I've gotten to moles, so now I need to convert my moles to mass. So what I need to do is now multiply by the molar mass conversion factor. I have moles on the top. I need to change it to grams. So my molar mass conversion is going to have the grams part on the top and the moles part on the bottom. Now my moles cancel out and I'm left with grams. Now I can run through and do the calculation. I plug that into the calculator making sure to use the special times 10 to the key so that my order of operations work out fine. And the answer that I get is 234.41 grams. If you forget the units of what you're calculating, go back to your original fractions and look for the unit that wasn't cancelled out. That's the unit that your final answer is in. Now am I finished? No, I need to check my sig figs. Go back to the original problem and find what is the lowest number of sig figs that you used. Here the number of molecules is 8.43 times 10 to the 24. That's three sig figs. The other value you used or the other two values you used were Avogadro's number but you have that to four sig figs, 6.022 times 10 to the 23. And the molar mass which you calculated from the values on the periodic table. Here you've calculated that to five sig figs. This is probably a good point to note that you should never let the molar mass become the thing that limits the significant figures in a calculation. Always calculate your molar mass to the largest number of sig figs that you possibly can so that it doesn't affect your calculation later on. Alright, so it was the molecules of ammonia that limit our sig figs. That's three sig figs. So that gives us 234 grams.