Another mass composition problem | Chemistry | Khan Academy
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Another mass composition problem | Chemistry | Khan Academy

October 18, 2019


I said I would get you a more
interesting mass composition to empirical formula problem,
one that doesn’t just have a straight-up 2:1 ratio. And so here it is. I have a bag of stuff. Or let’s call this a
bottle of stuff. Maybe it’s in its liquid form. And it happens to be 2.04%
hydrogen, 65.3% oxygen, and 32.65% sulfur. What is the empirical formula,
what’s our best stab at the empirical formula, of
this substance? So what we would do, like we
do in all these problems, let’s just assume we’ve got
100 grams of the stuff. So we have 100 grams
of the stuff. So we assume 100 grams. Let me
do that in a good yellow. So let’s say, assume I have 100
grams. How many grams of hydrogen do I have? If I have 100 grams total, 2.04%
of that is hydrogen, so I have 2.04 grams of hydrogen. I have 65.3 grams of oxygen. And I have 32.65 grams
of sulfur. Now, what we need to do now is
figure out how many moles of hydrogen is this. How many moles of oxygen. And how many moles of sulfur. Then we can compare the ratios
and we should be able to know the empirical formula. What is the mass of 1
mole of hydrogen? Let me write that. So 1 mole of hydrogen. Well we know what the mass
number for hydrogen is. It’s 1. And especially, the atomic
weight, also for hydrogen, if we were to take it on Earth. The composition, you
pretty much just find hydrogen nucleuses. If it’s neutral, it
has an electron, but it has no neutrons. So it has an atomic mass of
one atomic mass unit. So one mole of hydrogen. If you have a ton of hydrogens
together, or a mole of them, not a ton, I shouldn’t say, you
have 6.02 times 10 to the 23 hydrogens. Then you take hydrogen’s atomic
mass number in atomic mass units. And you say, well, it’ll
be that many grams of hydrogen, right? So if you immediately look up
here, if we have 2.04 grams of hydrogen, how many moles
of hydrogen do we have? Well, one mole is one
gram, so we have 2.04 moles of hydrogen. Notice, this said what the
mass of the hydrogen is. This tells us how many hydrogen
molecules we have. Remember, this is 2.04 times
6.02 times 10 to the 23 hydrogen atoms. Moles
of hydrogen. Maybe I should write
that down. So one mole of hydrogen. There you go. And then oxygen. One mole of oxygen. Oxygen’s mass number, in
case you forgot, is 16. Right there. Oxygen’s mass number is 16. So one mole of oxygen has a mass
of 16 grams. 6.02 times 10 to the 23 oxygen atoms
has a mass of 16 grams. So how many moles
do we have here? Let’s see. So if we take 65.3
grams of oxygen. And we have 16 grams per mole,
so you divide by 16. It equals 4.08125. I don’t want to get
too precise here. But let me just write that. 4.08 moles of oxygen. So I’m going to write that
in oxygen color. So I have 4.08 moles
of oxygen. And then finally, sulfur. What is sulfur’s atomic mass? The one thing I’ll never
forget about sulfur is its smell. There’s a city in Louisiana
which we used to drive to all the time — I think we had some
family friends there– called Port Sulfur. And they did a lot of sulfur
processing there. And lucky for the residents,
at least at the time– I apologize if they fixed the
issue– it smelled like sulfur, which smells
like rotten eggs. But anyway, one mole
of sulfur. So sulfur’s atomic mass
is 32 atomic mass units per sulfur atom. So a whole mole of it is going
to have a mass of 32 grams. So a mole of sulfur– not a mule. Maybe I should invent a new
unit called the mule. So a mole of sulfur is
32 grams. So how many moles do we have? We have a little bit more than
one, but let’s be precise here, because everything else is
a little bit of a decimal. So if we have 32.65 grams of
sulfur and we divide by the number of grams per mole–
divided by 32 grams per mole– we have 1.02 moles of sulfur. This was hydrogen up here. So here, you should hopefully
see a pretty good ratio, here. For every one sulfur atom– I
mean, the ratio worked exactly out and that’s because I did
this problem before. I actually made up this problem
before we worked, so I made it so the numbers
worked out. But one mole of sulfur, for
every mole of sulfur, so for every 6.02 times 10 to the 23
sulphur atoms, you have two moles of hydrogen, right? This ratio is 1:2. Two times 1.02 is 2.04. And then, for every one mole
of sulfur, you have four moles of oxygen. Right? Literally, if you multiply this
times four you get 4.08. So the ratio of hydrogen to
sulfur to oxygen is for every one sulfur, we have
two hydrogens and we have four oxygens. So the empirical formula
of this is H2. And then we have one sulfur. And then we have four oxygens. And this is sulfuric acid, one
of the things you would least like poured on you
most of the time. Anyway, hope you found
that useful.

Only registered users can comment.

  1. i paused the video at the very beginning when you only had the percentages listed. then i did all the work to find the empirical formula and i did all the steps and i got the right answer!!!…thank you so muchhh for being such a great teacher!

  2. @mcgarry11 It's useful to assume 100g, and then once the empirical formula (or the ratio of elements to one another) is found, then that ratio can be reapplied to any given amount. In this particular example, if a given amount of 500 were given, then once having obtained the empirical formula through assuming 100g, Sal could have reapplied that formula to 500g.

  3. @ppatbmw so did i and i completely agree with you. as do we all i dare say on behalf of the invisible crowd not bothering to comment =)

  4. could someone please explain why it ends up with H:S:O, or H2SO4, instead of say, O4H2S or SH2O4? this feels the most intuitive to me

  5. But what if you didn't have 100g? What would you do then? How do you figure out the grams of atoms in that?

  6. @lurkern It doesnt realy matter how you order it, but Sal wanted to order it the normal way that scientists do for Sulfuric Acid.

  7. @kat33333 It does matter because you'll need to know to find the # of moles of the element. If you have more than 100g for example, 123gram in a bag, then this is what you need to do:

    123g x 2.04%H (or .0204 ) = 2.51g to get the total grams of H in the composition.
    Same with O : 123g x 65.3% (or .653) = 80.32g
    S: 123g x 32.65% (or .3265) = 40.16g

    Now continue the problem by using these new numbers and each element's molar mass to convert grams to mol shown in video

  8. YOU ARE MY SAVIOR!! learning what my shitty gen. chem. teacher couldnt teach the class all damn semester…forever indebted!

  9. @goldensilverstar its still quiet easy even if you didnt use 100 grams. for example if the total mass of the mixture was 75.3, to find the mass of H O and S you just have to divide 75.3 by 100 then times by the percentage of the substance. eg, with sulfur it would be. 75.3 / 100 x 32.65 = 24.58 g. then you can do it with the O and H and see that all 3 values add up to 75.3. after that you just follow the rest of the instructions on this great video to figure out the empirical f

  10. @lurkern im pretty sure its because you always put a metals or hydrogen (not technically a metal) in front of all non metals and radicals. in this case you have hydrogen sulfate. (sulfuric acid) hydrogen goes before sulfate. and sulfate itself (SO4) i think they write the sulfur before the oxygen because there is less of it. i belive hydrogen can end up anywhere from first to last depending on what types of bonds its using. but please dont take my word for this. this is off my head.

  11. @goldensilverstar you fool. the percents for every mass composition problem would add up to 100. so the only thing you CAN make it is 100 grams.

  12. @goldensilverstar Why would you use another number for other than 100g? Since the percentage of X number of elements should add to 100 anyway, there is really no need to differ from 100g. No need to be rude, question before you demand something.

  13. @lurkern There is a order system called the Hill System which states that Carbon and Hydrogen are written first. If there is no carbon then you write it alphabetically. But since this is for Sulfuric acid, it is an exception to the Hill system. H2SO4 is an Ternary acid (pretty much anything that doesn't have "hydro" as a prefix and ends with "ic" – like Sulfuric) will always start with Hydrogen and end with Oxygen.

    Correct me if I'm wrong, anyone, but I think that's right.

  14. H2SO4 starts with an H because it's an acid. SO4[2-] is an anion. For acids H is written first, followed by an anion.

  15. You should be my Chemistry teacher. I watch your videos in my Chem class instead of listening to my teacher.

  16. In the previous video you were dividing with the atomic mass number of the element but in this video you swipe why?

  17. This video wasn't very useful. Your ratio was so simple and intuitive that you totally bypassed the working… which, in Uni, you need to show……

  18. What? At the end of the video you said no one wanted the acid to be poured on them MOST of the time as if people actually want that?lol

  19. Great work. Love the videos. I laughed when the ending of the previous video promised a better ratio and this one delivered. Thanks!

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