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We know what a chemical equation is and we've learned

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how to balance it.

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Now, we're ready to learn about stoichiometry.

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And this is an ultra fancy word that often makes people

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think it's difficult.

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But it's really just the study or the calculation of the

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relationships between the different

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molecules in a reaction.

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This is the actual definition that Wikipedia gives,

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stoichiometry is the calculation of quantitative,

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or measurable, relationships of the

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reactants and the products.

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And you're going to see in chemistry, sometimes people

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use the word reagents.

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For most of our purposes you can use the word reagents and

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reactants interchangeably.

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They're both the reactants in a reaction.

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The reagents are sometimes for special types of reactions

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where you want to throw a reagent in and see if

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something happens.

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And see if your belief about that substance is true or

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things like that.

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But for our purposes a reagent and

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reactant is the same thing.

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So it's a relationship between the reactants and the products

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in a balanced chemical equation.

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So if we're given an unbalanced one, we know how to

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get to the balanced point.

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A balanced chemical equation.

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So let's do some stoichiometry.

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Just so we get practice balancing equations, I'm

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always going to start with unbalanced equations.

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Let's say we have iron three oxide.

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Two iron atoms with three oxygen atoms.

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Plus aluminum, Al.

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And it yields Al2 O3 plus iron.

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So remember when we're doing stoichiometry first of all, we

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want to deal with balanced equations.

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A lot of stoichiometry problems will give you a

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balanced equation.

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But I think it's good practice to actually balance the

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equations ourselves.

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So let's try to balance this one.

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We have two iron atoms here in this iron three oxide.

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How many iron atoms do we have on the right hand side?

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We only have one.

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So let's multiply this by 2 right here.

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All right, oxygen, we have three on this side.

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We have three oxygens on that side.

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That looks good.

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Aluminum, on the left hand side we only have

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one aluminum atom.

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On the right hand side we have two aluminum atoms. So we have

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to put a 2 here.

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And we have balanced this equation.

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So now we're ready to do some stoichiometry.

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There's not just one type of stoichiometry problem, but

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they're all along the lines of, if I give you x grams of

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this how many grams of aluminum do I need to make

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this reaction happen?

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Or if I give you y grams of this molecule and z grams of

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this molecule which one's going to run out first?

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That's all stoichiometry.

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And we'll actually do those exact two types of problems in

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this video.

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So let's say that we were given 85 grams of the iron

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three oxide.

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So my question to you is how many grams of

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aluminum do we need?

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Well you look at the equation, you immediately

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see the mole ratio.

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So for every mole of this, so for every one atom we use of

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iron three oxide we need two aluminums.

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So what we need to do is figure out how many moles of

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this molecule there are in 85 grams. And then we need to

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have twice as many moles of aluminum.

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Because for every mole of the iron three oxide, we have two

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moles of aluminum.

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And we're just looking at the coefficients, we're just

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looking at the numbers.

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One molecule of iron three oxide combines with two

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molecule of aluminum to make this reaction happen.

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So lets first figure out how many moles 85 grams are.

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So what's the atomic mass or the mass number

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of this entire molecule?

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Let me do it down here.

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So we have two irons and three oxygens.

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So let me go down and figure out the atomic masses of iron

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and oxygen.

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So iron is right here, 55.85.

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I think it's fair enough to round to 56.

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Let's say we're dealing with the version of iron, the

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isotope of iron, that has 30 neutrons.

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So it has an atomic mass number of 56.

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So iron has 56 atomic mass number.

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And then oxygen, we already know, is 16.

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Iron was 56.

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This mass is going to be 2 times 56 plus 3 times 16.

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We can do that in our heads.

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But this isn't a math video, so I'll get

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the calculator out.

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2 times 56 plus 3 times 16 is equal to 160.

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Is that right?

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That's 48 plus 112, right, 160.

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So one molecule of iron three oxide is going to be 160

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atomic mass units.

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So one mole or 6.02 times 10 to the 23 molecules of iron

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oxide is going to have a mass of 160 grams.

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So in our reaction we said we're starting off with 85

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grams of iron oxide.

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How many moles is that?

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Well 85 grams of iron three oxide is equal

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to 85 over 160 moles.

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So that's equal to, 85 divided by 160 equals 0.53125.

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Equals 0.53 moles.

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So everything we've done so far in this green and light

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blue, we figured out how many moles 85 grams of

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iron three oxide is.

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And we figured out it's 0.53 moles.

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Because a full mole would have been 160 grams. But

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we only have 85.

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So it's point 0.53 moles.

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And we know from this balanced equation, that for every mole

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of iron three oxide we have, we need to have

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two moles of aluminum.

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So if we have 0.53 moles of the iron molecule, iron three

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oxide, then we're going to need twice as many aluminum.

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So we're going to need 1.06 moles of aluminum.

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I just took 0.53 times 2.

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Because the ratio is 1:2.

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For every molecule of this, we need two molecules of that.

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So for every mole of this, we need two moles of this.

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If we have 0.53 moles, you multiply that by 2, and you

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have 1.06 moles of aluminum.

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All right, so we just have to figure out how many grams is a

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mole of aluminum and then multiply that times 1.06 and

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we're done.

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So aluminum, or aluminium as some of our friends across the

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pond might say.

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Aluminium, actually I enjoy that more.

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Aluminium has the atomic weight or the

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weighted average is 26.98.

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But let's just say that the aluminium that we're dealing

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with has a mass of 27 atomic mass units.

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So one aluminum is 27 atomic mass units.

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So one mole of aluminium is going to be 27 grams. Or 6.02

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times 10 to 23 aluminium atoms is going to be 27 grams. So if

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we need 1.06 moles, how many is that going to be?

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So 1.06 moles of aluminium is equal to 1.06 times 27 grams.

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And what is that?

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1.06 times 27.

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Equals 28.62.

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So we need 28.62 grams of aluminium, I won't write the

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whole thing there, in order to essentially use up our 85

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grams of the iron three oxide.

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And if we had more than 28.62 grams of aluminium, then

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they'll be left over after this reaction happens.

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Assuming we keep mixing it nicely and the whole reaction

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happens all the way.

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And we'll talk more about that in the future.

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And in that situation where we have more than 28.63 grams of

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aluminium, then this molecule will be the limiting reagent.

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Because we had more than enough of this, so this is

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what's going to limit the amount of this

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process from happening.

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If we have less than 28.63 grams of, I'll start saying

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aluminum, then the aluminum will be the limiting reagent,

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because then we wouldn't be able to use all the 85 grams

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of our iron molecule, or our iron three oxide molecule.

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Anyway, I don't want to confuse you in the end with

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that limiting reagents.

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In the next video, we'll do a whole problem devoted to

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limiting reagents.

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