# Stoichiometry | Chemical reactions and stoichiometry | Chemistry | Khan Academy

We know what a chemical equation
is and we’ve learned how to balance it. Now, we’re ready to learn
about stoichiometry. And this is an ultra fancy word
that often makes people think it’s difficult. But it’s really just the study
or the calculation of the relationships between
the different molecules in a reaction. This is the actual definition
that Wikipedia gives, stoichiometry is the calculation
of quantitative, or measurable, relationships
of the reactants and the products. And you’re going to see in
chemistry, sometimes people use the word reagents. For most of our purposes you can
use the word reagents and reactants interchangeably. They’re both the reactants
in a reaction. The reagents are sometimes for
special types of reactions where you want to throw a
reagent in and see if something happens. And see if your belief about
that substance is true or things like that. But for our purposes
a reagent and reactant is the same thing. So it’s a relationship between
the reactants and the products in a balanced chemical
equation. So if we’re given an unbalanced
one, we know how to get to the balanced point. A balanced chemical equation. So let’s do some
stoichiometry. Just so we get practice
unbalanced equations. Let’s say we have iron
three oxide. Two iron atoms with three
oxygen atoms. Plus aluminum, Al. And it yields Al2
O3 plus iron. So remember when we’re doing
stoichiometry first of all, we want to deal with balanced
equations. A lot of stoichiometry problems
will give you a balanced equation. But I think it’s good practice
to actually balance the equations ourselves. So let’s try to balance
this one. We have two iron atoms here
in this iron three oxide. How many iron atoms do we have
on the right hand side? We only have one. So let’s multiply this
by 2 right here. All right, oxygen, we have
three on this side. We have three oxygens
on that side. That looks good. Aluminum, on the left hand
side we only have one aluminum atom. On the right hand side we have
two aluminum atoms. So we have to put a 2 here. And we have balanced
this equation. So now we’re ready to do
some stoichiometry. There’s not just one type of
stoichiometry problem, but they’re all along the lines of,
if I give you x grams of this how many grams of aluminum
do I need to make this reaction happen? Or if I give you y grams of this
molecule and z grams of this molecule which one’s
going to run out first? That’s all stoichiometry. And we’ll actually do those
exact two types of problems in this video. So let’s say that we were given
85 grams of the iron three oxide. So my question to you
is how many grams of aluminum do we need? Well you look at the equation,
you immediately see the mole ratio. So for every mole of this, so
for every one atom we use of iron three oxide we need
two aluminums. So what we need to do is figure
out how many moles of this molecule there are in 85
grams. And then we need to have twice as many moles
of aluminum. Because for every mole of the
iron three oxide, we have two moles of aluminum. And we’re just looking at the
coefficients, we’re just looking at the numbers. One molecule of iron three
oxide combines with two molecule of aluminum to make
this reaction happen. So lets first figure out how
many moles 85 grams are. So what’s the atomic mass
or the mass number of this entire molecule? Let me do it down here. So we have two irons
and three oxygens. So let me go down and figure out
the atomic masses of iron and oxygen. So iron is right here, 55.85. I think it’s fair enough
to round to 56. Let’s say we’re dealing with
the version of iron, the isotope of iron, that
has 30 neutrons. So it has an atomic
mass number of 56. So iron has 56 atomic
mass number. And then oxygen, we already
know, is 16. Iron was 56. This mass is going to be 2
times 56 plus 3 times 16. We can do that in our heads. But this isn’t a math
video, so I’ll get the calculator out. 2 times 56 plus 3 times
16 is equal to 160. Is that right? That’s 48 plus 112,
right, 160. So one molecule of iron three
oxide is going to be 160 atomic mass units. So one mole or 6.02 times 10
to the 23 molecules of iron oxide is going to have
a mass of 160 grams. So in our reaction we said we’re
starting off with 85 grams of iron oxide. How many moles is that? Well 85 grams of iron
three oxide is equal to 85 over 160 moles. So that’s equal to, 85 divided
by 160 equals 0.53125. Equals 0.53 moles. So everything we’ve done so far
in this green and light blue, we figured out how
many moles 85 grams of iron three oxide is. And we figured out
it’s 0.53 moles. Because a full mole would
have been 160 grams. But we only have 85. So it’s point 0.53 moles. And we know from this balanced
equation, that for every mole of iron three oxide we
have, we need to have two moles of aluminum. So if we have 0.53 moles of the
iron molecule, iron three oxide, then we’re going to need
twice as many aluminum. So we’re going to need 1.06
moles of aluminum. I just took 0.53 times 2. Because the ratio is 1:2. For every molecule of this, we
need two molecules of that. So for every mole of this, we
need two moles of this. If we have 0.53 moles, you
multiply that by 2, and you have 1.06 moles of aluminum. All right, so we just have to
figure out how many grams is a mole of aluminum and then
multiply that times 1.06 and we’re done. So aluminum, or aluminium as
some of our friends across the pond might say. Aluminium, actually
I enjoy that more. Aluminium has the atomic
weight or the weighted average is 26.98. But let’s just say that the
aluminium that we’re dealing with has a mass of 27
atomic mass units. So one aluminum is 27
atomic mass units. So one mole of aluminium is
going to be 27 grams. Or 6.02 times 10 to 23 aluminium atoms
is going to be 27 grams. So if we need 1.06 moles, how many
is that going to be? So 1.06 moles of aluminium is
equal to 1.06 times 27 grams. And what is that? 1.06 times 27. Equals 28.62. So we need 28.62 grams of
aluminium, I won’t write the whole thing there, in order to
essentially use up our 85 grams of the iron three oxide. And if we had more than 28.62
grams of aluminium, then they’ll be left over after
this reaction happens. Assuming we keep mixing it
nicely and the whole reaction happens all the way. And we’ll talk more about
that in the future. And in that situation where we
have more than 28.63 grams of aluminium, then this molecule
will be the limiting reagent. Because we had more than enough
of this, so this is what’s going to limit
the amount of this process from happening. If we have less than 28.63 grams
of, I’ll start saying aluminum, then the aluminum will
be the limiting reagent, because then we wouldn’t be able
to use all the 85 grams of our iron molecule, or our
iron three oxide molecule. Anyway, I don’t want to confuse
you in the end with that limiting reagents. In the next video, we’ll do
a whole problem devoted to limiting reagents.

## 100 thoughts on “Stoichiometry | Chemical reactions and stoichiometry | Chemistry | Khan Academy”

1. Isabella Chantrill says:

Great video, although I did wince when he stoichiometry like that…it's said "Stoke-e-om-etry" in my experience.

2. heretic says:

You are a life saver

3. Elijah Maurer says:

that's it, I'm gonna work at McDonalds for the rest of my life

4. J C says:

Where is the 85g coming from? Why did he say that Fe2O3 is 85g?

5. bts makes my heart goes uwu says:

Great video !thanks

6. steven compton says:

I am so going to stop watching this if I hear aluminium one more time
.
…. ugh it drives me nuts. Stop it.

7. steven compton says:

Seriously loving the videos though
.. just got disorientatated for a second…

8. Revival Lambert says:

9. afat rana says:

I like lecture of khan academy it's amazing thanks for helping us in such a way

thanks a lot .

12. Emill John says:

Where did he get that 1.06g of aluminum

13. Everett William Baxter says:

Im a bit confused on how he used unit conversion to get grams of Al? can anyone explain?

14. Djoanna Marie Dimapilis says:

Isn’t the atomic weight of Al should be multiplied to 2?

15. Itz Kim says:

Thank you!

16. amy2gr says:

As Greek everytime I hear the pronunciation of the stoichiometry word I have this urge to rip out my ears. The oi sound is one ( vowel) οι which is the same as η, ι, οι, υ, ει. In this particular words sounds like the English e as in (E)aster and it comes from the Greek word στοιχείο which is element. Note that both οι and ει are double vowels and sound exactly the same. As E

17. Aayush Sahni says:

There is God and then there is Khan Academy

18. Andy Gomez says:

Bless this man

19. rye says:

chem has the power to murder all teens

20. Vansh Rana says:

Stop it….

21. Josh Churchill says:

This man explained Stoichiometry better in under 10 minutes than my college chemistry teacher could in an hour and a half

22. Emma Carpenter says:

Oh my lord ALLOOOMUNIUM. I wanted to die. Other than that, great video

23. Edward Rojas says:

Can someone help me with this excersice, It says: During the complete combustion of a paraffinic hydrocarbon at 20 degrees Was got a volumetric contraction equal to 1/2. Find the weight of two mols of the hydrocarbon.

24. aldena suemia says:

3:20 'For every one atom of that' ???

25. aldena suemia says:

6:12 'Over 160 moles' ??? Really so we can freely interchange moles and grams now?

26. Lwazi Mathope says:

thanks

awesome

28. Noobish m-u-b-s says:

His voice is actually soothing😂

29. jesus palacios says:

30. Lauren's Life says:

I’m sorry but this didn’t help at all. I HATE stoich

31. loyughlty says:

So do we by heart the mol masses of all these elements because they obviously never provide that during the test

32. stewakiea says:

nice monotone voice makes me feel exicted

34. ABakedPotato says:

6:07 Wait wait, I'm missing something here obviously How did Fe2 O3 —> Fe3 O3?

35. wanna one's shit says:

my teacher is like eminem in teaching

what a waste of money

36. Erin G says:

I have an honors chemistry final in three hours and I now suddenly understand everything

37. Kaitlyn 22134 says:

my teacher didn't even properly teach this and its gonna be on the test tmr….

38. Yuli Dwsr says:

39. Brantuh says:

Thank you so much you just saved my life man! I would make myself a promise to never fall asleep in class again, however we all know I would never be able to do that

40. cameron burke says:

Can anyone tell me why we only found out the amount needed for one mole of Aluminium? Shouldn't the final value be multiplied by two or shouldn't the AMU be multiplied by two?

41. c0met 02 says:

THANK YOU!

42. Avrage Gamers says:

Good luck to everyone writing a quiz/test/exam tomorrow.

43. noose neryn says:

44. Erin Barela says:

great video. helped me to remember stoichiometry from 25 years ago to be able to help my 10th grader.

45. Jamie Knight says:

Did I just balance a thermite reaction…

46. Benster’s video stash says:

Careful, he's a hero.

47. NuclearBookery ASMR says:

The issue is that our chem teacher forces us to write it in a specific way that I don’t understand at all, if I could do it like this I’d be fine and I’d understand the questions 😂

48. GoxtedPlays says:

can you do Mole to mass please

49. Hala Aldossri says:

👍👍

50. Moshi Moshi says:

I learned this for 3 days and i learnt more from this vid. OHHH YEAH YEAH

51. Nedaa Mohammed says:

Sorry i canot understand why you times 56 with 2and 3 with 16 .I understand how to find the atomic mass. But atomic mass is what we times with numbers or what?

52. Nedaa Mohammed says:

I want to ask about something 1 mole of aluminim = 1 of atomic mass of aluminum ?

53. Aaron Cabiness says:

I can't hear it anymore. Everything else makes sound but this video.

54. - M A R I - 18 says:

Khan Academy I owe you big-time

55. Ryan Lutolf says:

I sure hope this is all I need to know lol

56. al.04x says:

British people getting triggered when he says Aluminum (In american accent).😂

57. Lavelle says:

I still wanna kno how he writes with a mouse

58. Jheantzen Calvin says:

Thank you so much for making this…This really helped me to understand stoichiometry

59. The Allied Powers says:

"Often makes people think it's difficult" On God chemistry teachers go out of their way to make things as complicated and difficult as humanly possible.

60. Logan says:

y’all better pray for my grades lol RIP test

61. aestheticia la says:

I'm watching this before the lesson because I'm usually behind

62. John Hoe says:

Just thought i would mention before i watch that my uni lecturer sent me here haha, must be doing some thing right.

63. MoaRam says:

I'm confused with the last part… Why did we use 1 aluminum x 1mol not the fixed equation where it has 2 aluminum.. So at the end why wasn't it 2al = 54g/mol x 1.06g

64. Debates Data base says:

Why law of mass conservation has to be obeyed during soichiometery calculations?

65. Arturo says:

At 5:45, why does he say Fe3?
The equation says Fe2, am I missing something? If so, what is it?

66. John Hoe says:

I'm confused i thought you need 2 al but at the end you only use 1 al to get your mole ratio can you please explain further ?

67. Brian Gilhooly-2021 says:

Excuse me sir I used this method and received a very poor grade thank you for nothing

68. EL CHINO says:

Have a test in 40 minutes lol

69. thog says:

Well this helped a lot. My teacher was doing it with dimensional analysis and it seemed complicated but this cleared it up and makes it look easier

70. Midori Neko says:

Random guy showing Stoichiometry definition using Wikipedia

RESEARCH PROFESSOR WANTS YOUR APA FORMAT CITATION

71. yikesamanda says:

i'm currently failing chem with an F and i've got a test on this tomorrow and i couldn't pay enough attention to understand the lessons throughout the month that it took my teacher to teach it and after this video i STILL can't grasp the concept so uh rip my grade guess i'm definitely taking that final

72. bruh says:

god bless sal khan.

73. Peroroncino says:

I missed out on 850 energy points because I watched this video here instead of the KA website >:(

74. Nandina Saikia says:

Understood….thnks a lot

75. Ahmet ALTINDAS says:

thank god there are people like this who saves us from our chem teachers

76. Kobe Russell says:

I absolutely hate you. you said (al-you-min-ee-um)

got a guide due tomorrow and a quiz, hope I get all this memorized

78. Sarth Diskalkar says:

good luck to everyones chemistry finals aka which is why I am here cramming for a good grade

79. 2 GuD says:

Thanks man

80. Swiggity Swooggity says:

Thank you so much, I might not fail my chem final now

81. Binkular Man says:

I have chem finals tomorrow morning lol

82. Bob da builder says:

time to drop chem?

mic quality: pewdiepie

84. Sethuraman Ramachandran says:

Thank you so much I will surely pass my test Sal

85. Mer-Hamid Ali Abuat says:

2:36

86. catirerubio says:

Khan Academy is great, but this video loses me.

87. Kamallistic Kids says:

patision for stoichiometry be changed to stonkiometry for no particular reason
I
I
V

88. kenneth torre says:

How to get 85g.?

89. Rhemi Ruth says:

Im legit gonna cry ughhh

90. Epic Sax Guy says:

Sal is the god that high school students need

91. Kiersten McComas says:

Sal can you please just be my chem and math teacher! Mine don’t teach well at all 😭

92. HuzaiFa Javed says:

Sal, You made it real Hard tbh for a revisor
As one can do Cross Multiplication by Taking Mr and given Grams
like
Mr Of Fe203 X Mr Of Al2
85 grams x

93. Gwenyth Patrice Magpatoc says:

1.60 how did you get that?

94. Sanele Khuboni says:

finally some1 spoke sense..thank uu so much😍🤗

95. NC car guy says:

My chemistry teacher said if we don't get this then we will most likely fail the class thank God for Khan academy

96. Mr. Narrator says:

Lazy teachers😑

97. Big Juan McCarthy says:

that 2009 calculator xD

98. Sally May says:

I hate this part of chemistry

99. aiaru says:

it’s so funny how here are comments from 3 years ago where people are like oh no i’ve got an exam tomorrow and etc. and they probably don’t even remember those tests

100. Belal Ayman says:

guys, you have 2 aloomonom atoms