Learn about Junior Chemistry, The Mole 3, in this comprehensive video by bannanaiscool.
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Rob Lederer: We ask how many moles again like before but this time, we have some zinc chloride and we've got 2.6 grams. 2.6 grams, how do we find the moles? So we need a number to multiply by here that's got grams in it and moles, that's the molar mass, that's the number in the periodic table for each of these elements, we can add them together. So 2.6 grams of zinc chloride, well we need to multiply by a ratio that gets rid of the grams of zinc chloride, which is here and leaves us with moles of zinc chloride because that's what we want in the end, so we need the molar mass. Zinc chloride is one zinc, 68.35 -- 65.38, that's it and here we've got two chlorines at 35.45 grams per mole each. You add 35.45 times two, plus 65.38 together and you are going to get, here is what I get, I get 136.28, so 136.28 grams of zinc chloride every time you have one mole of it. When you multiply this times one, don't do that in your calculator, that's just embarrassing, and then divide by 136.28, you get as an answer 0.019 moles of Zncl2. Now listen, that cancels that, grams of zinc chloride, two significant digits, five here, that's why I kept two in my answer. You look at this and you're saying, that two, what about these numbers in front? Zeros in front of the first natural number that occurs never count as significant because you can write this number as 1.9 times 10 to the negative two. You see, if you are turning into scientific notation, these numbers are just lost, so they are not significant. So therefore there is your two significant digits and that's your moles in chloride there. This math is lovely, let's do something else, lovely. You've asked how many moles but let's ask how many grams do we have in this? 1.973 moles of iodine, so this is how many moles of I2 we have? That more than 6.02 times 10 to the 23 of these? Yeah because that's more than one mole, we've 1.973 moles out there. How many grams do we have? That's the question, it's math. Well, we need a number that will cancel out moles and leave us with grams. Grams, moles, molar mass, top right hand corner of the table, Iodine's is 126.90, but there is two of them in the formula, so that means then that when we multiply here and get rid off the moles of I2, since all unit cancelation is beautiful, moles of I2 is going to cancel. We're going to leave ourselves with grams of I2, but what's the ratio between these two? There is 253.80 grams of I2 everytime you have one mole of I2, because that is the molar mass doubled from what it is on the periodic table, right? Okay, so now moles cancel here, you left with grams. And when you do the math here, you get 500.7 grams of I2, is that right? Is that the number? Then we want here one, two, three, four significant digits, five here, multiply and divide and keep the least number. One, two, three, four 500.7, it's beautiful; this is getting good, right? It's easy. Chemguy, Chemguy, you've been throwing fast balls over the plate, I've been knocking them out of the park, this is great stuff. Yeah, now I throw you the curve ball, I throw you the slider, and now you got to get ready to handle questions like this. How many atoms are there if you have 5.5 grams of iron? Now it look like a 5.5 grams of iron, you determined grams in moles, right? But that's it, you know the ratio, that's the molar mass ratio to get to turn grams into moles, but then how do you get atoms? Because you know another ratio, you know how many atoms are, are in a mole, 6.02 times 10 to the 23. So you see if you are patient and you do this unit cancelation now in two steps, you're going to get the answer, watch. 5.5 grams of iron, if you the molar mass of iron, which is 55.85 grams of Fe for everyone mole of Fe, that's cancel of Fe and you've got moles of Fe, but you don't want moles of Fe. So guess what, do another ratio and get rid off the moles of Fe and keep atoms of Fe. You see, it's all ratios, it's all cancellation of formulas to me, am I right? But this is