does this caption encompass the video Calculate quantities of a
substance or its relative
number of particles using
dimensional analysis and the
mole concept. all right good morning everybody welcome to the very first uh video for um ap chem and this is unit 1.1 so let's get started so i always like to start my powerpoint presentations with a little factoid this is not suggestive that you should fall asleep but i figure some of us fall on either side of the spectrum some of us are the type that your head hits the pillow and you're out and others of us it takes us 30 minutes to fall asleep anyway so unit 1 is all about atomic structure and properties and the first subunit 1.1 is all about the mole and molar mass so what is the mole well i always relate it when i'm teaching the mole for the very first time to people that have never heard it before i say well if a dozen is 12 the mole is a certain number okay so there are certain words in our everyday life that mean certain numbers like if you go to costco you'll see on the sides of the boxes you'll see the word gross and it doesn't mean like ew gross it means there's 144 of whatever that item is in that box okay a ream of paper has 500 sheets so how many things are in a mole 6.02 times 10 to the 23rd okay that's the number of things in a mole okay and we're going to relate this to the term molar mass okay and this handsome gentleman in front of you that is amadeo avogadro okay and that number 6.02 times 10 to the 23 is sometimes referred to as avogadro's number he didn't discover it but they just they named it after him okay so we're going to talk a lot about molar mass in chem we're going to use that those numbers almost every single day so what is molar mass okay well if we have one let's use sulfur as the example in front of us if i have one single atom of sulfur okay it will have a mass of 32 amu's that stands for atomic mass units okay where did the number 32 come from you can look on your periodic table and look at sulfur's atomic mass so one atom of sulfur has a mass of 32 amu's but we are not dealing with problems in just individual numbers of atoms we're dealing with with substances that are in much greater quantity so if i have one atom of sulfur it's 32 amuse if i have a mole of sulfur atoms 6.02 times 10 to the 23rd it will have a mass of 32 grams okay that's molar mass the mass in grams of one mole of that substance and in this picture in front of you you have two different substances sulfur is on the left potassium dichromate is on the right this picture is showing you one mole of both of these but you'll notice the number and you know quantity look they look very different okay and that's because each individual particle weighs differently okay one sulfur atom has a very different mass than one formula unit of potassium dichromate so you can see there that potassium dichromate has a much higher molar mass its particles simply weigh more okay so it's like if i had a mole of feathers 6.02 times 10 to the 23rd feathers that would have a certain mass if i had 6.02 times 10 to the 23rd dump trucks that would have a much much greater mass i want you to notice the units grams per mole okay these that's the uni those are the units for molar mass so the numbers that we pull off the periodic table we're going to instead of talking about atomic mass or formula mass we're going to talk about molar mass from now on and i want you to use those units grams per mole okay now for those of you that have been in my class before this diagram is going to look familiar to you we're going to make conversions between different quantities okay and this is my little graphic organizer that i use to help me remember what goes where okay what conversion factors to use all right so let's say we wanted to convert the mass of something into moles or vice versa we have a number of moles and we want to know how many grams that is okay or maybe we want to convert from number of particles atoms molecules ions formula units that's just a fancy term that means number of ionic compounds if we wanted to convert between that and a mole we can if we want to convert between volume and moles or any of these other quantities we can do that the point of this graphic organizer is to say that when you're converting you must convert to moles first before you do anything else so what are the conversion factors we use to get back and forth between grams and moles and it doesn't matter which direction we're going grams to moles or moles to grams we use the molar mass off the periodic table [Music] if we're going between number of particles and moles we're going to use avogadro's number if we're going to talk about volume we're going to use this conversion factor of 22.4 liters however that is only for gases at stp we're not going to talk about that today we'll get to volume later on so let's look at some practice problems okay so i'm going to do this first one with you right i've got two more that i want you to stop the video and try on your own okay but this is just review okay from chem one so it says how many grams of lithium are in 3.5 moles of lithium so the first thing i'm going to do and this is refreshing your memory on dimensional analysis okay so i set up what i call a t chart okay remember i'm starting out in moles of lithium so i'm going to put that same unit moles of lithium on the bottom so it cancels out and according to that graphic organizer if i'm starting in the middle circle if i'm in moles my given number is in moles i can convert directly to grams and let's refresh our memory on what numbers go where one always goes with the number mole for now okay and according to that graphic organizer what goes with grams what was the conversion factor the molar mass off the periodic table okay so moles of lithium are going to cancel out we multiply across divide by any number on the bottom and i'm going to round that to three significant figures and there's my final answer okay so there are two more problems here okay i want you to pause the video try these two on your own and then come back and you can check your answer okay so you can see my answers here for numbers two and three and unlike number one number two has the molar mass in the denominator now how did i know to put it there because i started out in grams of carbon dioxide so i need grams of carbon dioxide to be in the denominator and that's where the molar mass goes now number three was a little bit more of a challenge because it wasn't a one-step problem it was a two-step problem i knew that it was going to be a two-step problem though because if i used that graphic organizer i could see that going from grams to formula units you have to convert to moles first and that's what i've done here in this first step here and then and only then can i convert convert to formula units so it's a it's a two-step problem okay but i didn't wasn't good about canceling out my units here but at the end of the day the only unit that is left is formula units of sodium chloride so that is where we will stop that is the end of unit 1.1 and i hope you've learned a little something see you next time