 Okay. Today we're going to be working on a smart board presentation to do some sample problems and this first set is going to be sample problems about working with moles. And so hang with me, there's going to be windows popping up for the calculator and for the periodic table. So we'll be working in this white space plus these pieces of information will be popping up. The periodic table that we'll be using in these problems is the same one that's online is an interactive periodic table so as you touch an element it'll come up with the atomic number and the atomic weight. You will notice that this periodic table gives the values to three decimal places. Some periodic tables will round those off to two decimal places. In fact one of the periodic tables we use in some of the interactive problems that you'll be doing the samples are rounded off to the second place. This periodic table I'm going to use the samples today taking them out to the third place as this periodic table does. You can use either one to be aware that the rounded off numbers will be slightly different than the numbers that go to three places. So that out of the way let's look at working with moles and we're going to start with individual elements. And the first individual element we're going to look at is magnesium. Chemical symbol MG. So our question is what is the gram atomic weight? Here's your periodic table. Here we see magnesium. We touch it. It comes up as having the weight 24.305. 305 gram atomic weight. This is the weight in grams of one mole of magnesium. We're about phosphorus. It's our periodic table. We find phosphorus as atomic, gram atomic mass of 30.947 grams. So one mole of phosphorus is equal to 30.947 grams. So then what would be the weight of three moles of calcium? So we need to call it periodic table and find what the weight of one mole would be. Here we have calcium. So we see the weight of one mole is equal to 40.078 grams. So for three moles, we take the 40.078 grams per mole times three is equal to 120.234 grams. Now we've been doing things by proportion. So if you don't just see that it's just times three, you could say 40.078 grams is to one mole as how many grams are there to three moles. So we do the cross multiplication thing. So we get 40.0740.078 grams times three moles is equal to how many grams times one mole. So we rewrite that in the verbose side by one mole. So the unit counts out and one into three is three. So we wind up with the same equation. 148 grams times three is equal to 120.234 grams. Okay, so here's another problem. What is the weight of 250 millimoles of sulfur chemical symbol s? Okay, so let's call it the periodic table. Look at the chemical symbol s and we see that the atomic mass for sulfur is 32.065. So s 32.065 grams mole. Now there are 1,000 millimoles. One there's that metric system the milli right meaning 1,000. So there are 1,000 millimoles per one mole. So if we say 32.065 grams per 1,000 millimole is equal to how many grams per 250 millimoles. So we just replaced one mole right with 1,000 millimole. So we keep these in the same unit. So now we can solve this by our cross multiplication. So we can say 32.065 grams times 250 millimoles right is equal to how many grams times 1,000 millimoles. So we can divide both sides by 1,000 millimoles. 32.065 grams times 250 millimoles divided by 1,000 millimoles is equal to our unknown grams. Okay so the units are canceled we have 250 divided by 1,000. So this simplifies to 32.065 grams times 0.25 is equal to our unknown grams right and that becomes 8.016 grams is equal to 250 millimoles of sulfur. So with individual elements it's a matter of keeping track of your units and finding on the periodic table that individual element. Now we're going to look at some molecule. So the molecular weight is equal to the sum of all the atomic weights of the elements in the molecule. So let's look at an example. Let's find one mole of calcium chloride. So one let's identify the elements calcium and chloride and let's say there's one atom of calcium and there's two atoms of chlorine right. So now two we'll go to the periodic table and we'll find out that alone calcium. Okay let's try that again let's see if the periodic table behave here we've got calcium which is 40.078 grams per mole and chlorine and chlorine which is 35.453 grams per mole. So let's put those here 0.078 grams and 35.453 grams okay and now we're going to multiply by the number of atoms for each element. So for calcium that's one for chlorine that's two so that's the third thing we're going to do and so we're going to get 40.078 grams per calcium and 70.906 grams for chlorine and the fourth step we're going to sum those up and get 110.984 grams. So for calcium chloride the weight is equal to in 10.984 grams or 110.984 grams that's for calcium chloride. Now let's turn what is 100 millimoles of sodium chloride right what is the weight of 100 millimoles of calcium chloride right so the formula is NaCl so we identify the atoms of the elements right we have one of each so we need to find the mass so let's cross our fingers on this periodic table all the chlorine is still up there so it's still 35.453 here sodium is 22.990 right and now we'll multiply those by how many in this so there's just one of each so that makes easy math that's 22.990 grams and 35.453 grams so that comes out the molecular weight of sodium chloride is 58.443 grams or one mole okay so how about 100 millimoles okay so remember there are 1000 millimoles and one mole okay so if the there's 58.443 grams per mole and we're going to we're going to express that get my pin going in it as 1000 millimoles for how many grams in 100 so this is our cross multiplication divide yeah so it's 58.443 grams times 100 millimoles is equal to how many grams right so if we bring and divide both sides by that 1000 is going to just cancel out of that side those would be grams times 100 millimoles divided by 1000 millimoles equal to how many grams right again the millimoles are going to cancel out 100 over 1000 is 0.1 so that's 58.44 grams times 0.1 is equal to 5.84 grams so there are some practice problems some notion on the strategy for working on problems dealing with moles in the next session we're going to deal with molarity so you know follow through this video work some sample problems there's also a kind of interactive tutorial on these kinds of problems so you know the way you're going to hang in there as I keep saying this it's just the work problem to get used to it thanks and watch the session on molarity