A couple of videos ago we

figured out how to go from the empirical formula to the

mass composition. And all I mean by that, mass

composition, is that you could start with the empirical formula

H2O, which is also its molecular formula. And then we were able to figure

out what percentage of this was water and what percentage of this was hydrogen. The way we did it is we said,

OK, oxygen’s mass is 16 atomic mass units. Hydrogen is one. The mass of the entire molecule

is 2 times hydrogen. 2 times 1. Plus 16 for that one

oxygen, so it’s 18. And then we said, the

composition of oxygen is just 16/18. So oxygen is 16/18. I don’t have my calculator

anymore. I think the number was something

on the order of 88% or 88.9% oxygen. So now let’s see if we

can go the other way. Let’s see if we could start

with mass composition, the percent composition of the

different elements, and then go to empirical formula. And I think the first

video I made, I spelled empirical wrong. Which is good reason for me

not to do spelling videos. I think I spelled this with

an e right here, because I pronounce it empricial. But it’s spelled with an i. Empirical formula. So back to the empirical

formula. So let’s do some exercises

here to see if we can get someplace. So let’s say that I start off

with– I need space, my periodic table is right

down there. Let’s say I am told that

I have a bag of stuff. And 75% of that bag, let’s

say it is mercury. Hg. And say that the other 25%

of the bag is chlorine. So the question that I’m asked

is, what is the empirical formula of the stuff that I have

here, assuming that it’s all one molecule or one

type of molecule? So what is the empirical

formula? So the way to think about this

is, let’s assume that you have 100 grams of this stuff. Right? And the reason why I’m picking

100 is because 100 is a useful number to deal with. So if I have 100 grams of

the stuff, all right. So let’s say I assume that you

have 100 grams. If you have 100 grams, how many grams

of mercury do you have? Well that means that 75% of

that, that means you have 75 grams of mercury, Hg. And that means you have

25 grams of chlorine. You might be saying, where did

get these numbers from? Well, I got the numbers because

I assumed 100 grams. I could have assumed 112 grams

or 7 grams, but those would have been harder numbers

to work with. So I’m just assuming 100 grams.

So the question is, in 75 grams of mercury, how many

moles of mercury do I have? So I need to convert from

grams to moles. Let’s go the other way. One mole of mercury– this is at

least how my brain thinks– let me pick a suitable color–

so one mole of mercury so 6.02 times 10 to the 23

mercury atoms. What’s the mass of that? Well it’s equal to whatever

mercury’s mass number is in grams. So let’s go look up

mercury’s mass number. So mercury is a transition

metal. It’s one of the few metals

that are liquid at room temperature. Right there we have mercury. Its mass number, let’s

pick the 200. It has a mass number of 200. Obviously, there’s some mercury

out there that has a mass of 201. But for simplicity, let’s

say it’s 200. So 1 mole of mercury is 200

grams. How did I get that? Mercury’s mass number. One atom of mercury has a mass

of 200 atomic mass units. So one mole of mercury– 6.02

times 10 to the 23 mercury atoms– has that many

grams of mass. So instead of 200 atomic mass

units per atom, we have 200 grams per mole of that atom. It’s an easy calculation. You just look up it atomic mass

number and say it’s that many grams. You have a mole

of the substance. Fair enough. And what about one

mole of chlorine? We do the same exercise. We go down to our

periodic table. Chlorine has a mass number

of, let’s say, 35. It has various isotopes

on this planet. But let’s say the mass

number is 35. So one chlorine atom weighs–

well now, we shouldn’t say weighs, we have to

be very careful. One chlorine atom has a mass

of 35 atomic mass units. So 6.02 times 10 to the 23

chlorine atoms, or one mole of chlorine atoms will

have a mass of 35. 35 grams. So how many moles of

mercury do I have? Let me get my calculator

going. So I have 75 grams of mercury. One mole of mercury would have

a mass of 200 grams. So I could just take 75 divided by

200 and it tells me how many moles I have. 75 divided

by 200 is equal to 0.375 moles of mercury. How did I figure that? One mole of mercury would be

200 grams. I only have 75 grams of mercury, so I have a

small fraction of it, roughly 0.375 moles of mercury. I’ve just expressed the number

of atoms of mercury I have. Moles tells me it’s 6.02 times

10 to the 23 atoms of mercury. So I have 0.375 times Avogadro’s

number of mercury atoms. Fair enough. Now let’s do the chlorine. I have 25 grams of chlorine. One mole of chlorine

is 35 grams. So I have 25 over 35 moles. So what’s that? So 25 divided by 35 is equal

to 0.714 moles of chlorine. Now these numbers, they’re

not exact. Because if you take twice this

number up here, you don’t get twice this number right here. If you get twice this,

you don’t get this. But this is roughly twice as

many moles of chlorine as I do of mercury. So that tells me that for every

mercury atom– remember, moles is just a number– for

every mercury atom I have, I have two chlorine atoms. So the

empirical formula here is for every mercury I have

two chlorines. I have two chlorines,

right there. And the numbers didn’t work

out almost exactly right. Especially in the real world,

when you’re actually trying to figure out things empirically,

your numbers seldom will. And maybe there’s some random

other things running out there in terms of other things that

are contributing to the mass. But this is close enough to know

that the ratio of mercury to chlorine is roughly 1:2. Right? This is a number. For every one chlorine atom,

you have roughly two. You have twice as many. I guess the other way of

thinking about it is you have twice as many chlorine

atoms in the bag as you have mercury. Roughly. Although this is a

little bit more. This is close to 0.75. But it’s close enough for you

to know that you’re dealing with mercury chloride. Right there. Let’s do another one of these. Let’s see how the numbers

turn out for this one. Let’s say you have another

bag that is 9% magnesium. And let’s say the remainder of

the bag, 91%, is iodine. 91% is iodine. So the way to do all of these

is, you do the same thing. Assume you have 100 grams. So

if you have 100 grams, of which you have 9 grams

of magnesium. And you have 91 grams

of iodine. And then figure out how much a

mole of magnesium– what would be the mass of a mole

of magnesium and then a mole of iodine. So let me write here. One mole of magnesium. And we want to figure out

one mole of iodine. Let’s figure those out. So magnesium’s mass number is,

let’s just go with 24. Let’s say we have the

isotope that’s 24. Magnesium is 24 and since

we’re already down here, what’s iodine. 127. It’s 127. Iodine is one of the halogens. So let’s see. You have 127 and you have 24. Let me write those down. So you have 127 and

you have 24. So one atom of iodine has a mass

of 127 atomic mass units. So a whole mole all of it, 6.02

times 10 to the 23 iodine atoms will have a mass of 127

grams. You just take the atomic weight, or the atomic

mass, and the mass will be that many grams when you have

this many of the atom. So then one mole of magnesium

will be 24 grams. So now we just figure out how

many moles of each of these we have. We have 9 grams

of magnesium. So what fraction of

a mole is that? A mole is 24 grams. So

this is equal to 9/24 moles of magnesium. And what is 9 over 24? 9 divided by 24 is

equal to 0.375. So this is equal to 0.375, which

is similar to what we had in the last problem when

that 0.375 showed up. And 91 grams of iodine

is what? 91 grams of iodine. Well, in a whole mole of iodine

you’re going to have 127 grams. So we have 91/127

moles of iodine. And what’s 91 over 127? I have a feeling we’re going to

have very similar numbers. 121 divided by 127 is

equal to 0.716. I should do another problem with

better ratios than this. These have the same ratios as

the last problem, 0.72, roughly, moles of iodine. And this is moles

of magnesium. So we have roughly twice the

number of iodine atoms as we do of magnesium atoms. Right? For every one magnesium atom

we have roughly two iodine atoms. I know this isn’t

exactly 1:2, but it’s pretty close. So the formula here is

magnesium iodide. Right there. And that’s the empirical

formula. We don’t know, maybe in every

atom maybe you had two magnesiums and four iodines. We don’t know. All we know is that the

ratio of magnesium to iodine here is 1:2. In the next video, I’m going to

try to look for ones that have more interesting ratios,

because I don’t want to do two problems that both have

the same ratio. But hopefully you found this

a little bit helpful.

Very good video.

I like the way you have an easy going manor which makes it easy to understand what you are teaching.

This will be great for my son whom I am going to be home schooling this year.

Thank you for the effort your expending on these videos. 5/5

there is 100 g of substance lol jk idk

really great stuff, these vid's are a life saver! (Y)

"some mercury out there has a mass of 201" the truth is that only some mercury has a mass of 200 most of it has more or less.

needs to be an edit on the very last worked problem. easy mental slip up…but if there were .375 moles of Mg and .72 moles of I, then the empirical formula should be Mg(sub 2)I…since it is roughly 2 Mg to equal 1 I…. just wanted to make a note of that. This guy is an amazing teacher…no complaints from me 🙂

thankk you soo much for the videos! they're really easy to follow and REALLY helpful! 🙂

hey sal can you do a video about dimensional analysis

WOOT I GET IT

WIN WIN WIN

you know that you are good at teaching. thanks a lot!

@xxkora NH3's molar mass is

1×14.01g/mol

3×1.01g/mol <<found on periodic table

———–

17.04g/mol

n=m/M

M=17.04g/mol

m=100g

n=100g/17.04g/mol

n=5.87mol

i think thats what your asking for =)

ur arrows look like penis, other than that this has been helpful

this helped me so much and i used it to study for my test.. thanks for the great videos

your voice is so sexy. it sounds like penn badgely!

can you do this with a compound?

@rAchellelyn08 You look at the "mass" of that element in the periodic table

@rAchellelyn08 true if he would chose a different value for 100 it would be easier to get it :/

@ARMOROID5000 I hope you'll teach your kid the proper usage of "your" and "you're"??

I'm preping for HL IB chem, having never taken chem before. I'm just going through these videos one after another and it's like taking a year of chem.

is there some coincidence you always start out with getting things in bags?

This is amazing. Thank you for helping me (hopefully) not fail another test. Please come teach here at UMM?

Noooooooooooooo the caculator is gone 🙁

Khan can u make curry appear out of this magic bag of yours too?

His bag has fucking everything

his voice keeps me interested…

@goldensilverstar I know and his arrows look like penises

This video saved my life.

@7RaPeKiNg7 From what I understnad goes like this, you knwo that the number of moles equals as the grams of substance divided per the molecular mass. And he did that and got the number of moles. So he knew in 100 g of substance you have 75 grams of Hg, using the equations you obtain number of moles equals the mass witch is 75 grams divided per the molecular mass of substance which is it 200, and you obtain the number of moles.

use mol/g

This has helped SOOOOOOOO much!!!!!!!!!!!!! Thanks!

Yes. He could teach you to spell college.

omg.. this is the longest way to get the answer lolll if u have the periodic table u cud figure out its molecular formula and than empirical formula.. noo lets jus throw moles

Actually, for the exam I sit it's paramount that you use the moles technique, it helps when you advance further into chemistry.

HOLY FRAK IT FINALLY MAKES SENSE.

I'm a dude and I dont feel funny in saying I love you bro. You gettin me through college !!!!!

Hey guys, I know this wont be liked, but I've created study guides on chemistry, organic chemistry, physics, and biology that I've made easy enough for anyone to understand, yet thorough enough for use for high school, college, and MCAT review. My goal is to change a scientific education for everyone regardless of income level, scientific background, etc. I'm also keeping the price at 9 bucks

I can't post a link, but you can find all my books on amazon by searching "konstantinos papadopoulos"

Actually, he probably couldn't 🙂 he makes mistakes in his video as well as well.

Other than looking and seeing that one amount is aprox. twice or 3 times, is there a more accurate way to deal with those weighted numbers? Like dividing by the smallest one so it is equal to 1, and hoping the others either clearly do round up to the next whole number or clearly don't?

Dude. I cannot believe that your two examples of I and Mg, Hg and Cl had the same ratios…made me laugh out loud.

I'm fucked for this exam

funny how he helps me more then my teachers do

*hail

And God dammit.

awesome

is it just me or do your arrows => look like penises? I can't say I don't fantasize about em

I Like all of your videos sir! But your vids would be a lot much more better if you brought your calculator. Cuz you don't have it like, all the time.

I love his house, too.

#manor

Grade 11 chemistry is retarded because you have to round so much the answers arent even accurate

The rounding off is driving me nuts.

Sal stop drawing dicks everywhere!

Nice video man!

can you explain why, even though the molcule contains more grams of Hg than Cl, there are more moles of Cl than Hg and therefore more atoms of Cl than Hg?

got an exam tomorrow… the best procrastinator in the world I think so!

furthermore, the way my teacher helped us calculate empirical formulae is much more different than this… but this helps me fill in so many gaps in my knowledge i forgot about.

Shouldn't it be Hg2Cl?

If the word atom didn't get left out of the expressions this wouldn't be so difficult for so many students.

It is very clear what one means with "a dozen atoms of mercury". We are counting atoms, and there are 12 of them (a simple substitution: "a dozen"=12).

When it comes to moles, however, it becomes "1 mole of mercury", and the fact that we are counting atoms stops being obvious."[One mole of] atoms of mercury" = "[6.022 * 10^23] atoms of mercury" would make things a lot easier until the student gets familiar with the concept.

When you say you have 25% or 9% of something "in a bag", you should be clear that you mean 25% or 9% by mass, not 25% or 9% by volume, because when we talk about having an "amount" of something, we often mean we have a certain amount by volume, such as one liter or one gallon, but in this case we should be clear that we're talking about percentages by mass or weight, such as 25% of a one kilogram or one pound bag of stuff.

Good teaching

Just putting this out there – Mercury's most common isotope is 202, and it's second is 200. Because of the fact that it has 7 stable isotopes, it's average is 200.5.

The word 'roughly' kills me in chemistry.

mass/molar mass = moles

that's pretty much all that is required to solve molecular mass composition problems and of course SI unit conversion i've never seen amu used molgmol^-1 is used instead.

<sigh> Too bad you can't use the speedup feature on mobile youtube…

Americans don't use this table?It's in Dutch, but you geet the point. https://upload.wikimedia.org/wikipedia/commons/thumb/f/f5/Scheikunde_chemisch_rekenen_overzicht.png/680px-Scheikunde_chemisch_rekenen_overzicht.png

So do this mean that 1 mole = 1 atom = amu??

8 years later and this video is still saving lives. Im in Uni and I still watch your videos lmao

Did he just ASSUME Mercury's isotopic mass number?!?! You can't assume that! 😛

God bless you, Sal.

Can someone explain why Cl has a smaller percentage but more moles?