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• Stoichiometry is a kind of calculation in chemistry. It’s sort of like bookkeeping.

• Stoichiometry lets you figure out how much of a substance is used up or produced in a

• chemical reaction. You need two things to do stoichiometry:

• 1. A balanced chemical equation (balanced is absolutely essential here)

• and 2. Some measured values (the amount of some of the chemicals involved in the reaction).

• You can use these 2 things to solve for whatever is unknown - the mass or volume of

• one of the other chemicals in this balanced chemical equation. It’s really common to

• measure the amount of one reactant, in order to figure out how much product you expect

• to get. Or, given how much of one reactant you have, you might want to figure out how

• much of a second reactant to add so you don’t have any left over.

• Stoichiometry works because of the Law of Conservation of Mass. Chemical reactions

• don’t create or destroy matter, so say you have a certain number of atoms of oxygen at

• the start of a reaction, you will have the same number of atoms of oxygen at the end

• of the reaction. You can recombine the atoms with other atoms, but you can’t destroy

• them, and you can’t change the atoms into some other kind of atom.

• The balanced chemical equation tells you how the atoms combine in very definite ratios.

• It’s like a recipe. Let’s say you want to make grilled cheese sandwiches. You take

• 2 slices of bread and one slice of cheese to make one grilled cheese sandwich.

• Another way you could say this is for every 2 pieces of bread, you need 1 piece of cheese

• to make 1 grilled cheese sandwich. Believe it or not, this is just like a balanced

• chemical equation. When you see the equation:

• N2 + 3H2 → 2NH3

• It’s the same as saying it takes 2 atoms of Nitrogen + 6 atoms of Hydrogen to make

• 2 molecules of NH3. The equation also says 1 molecule of N2 + 3 molecules of H2 produces

• 2 molecules of NH3. Another way to read the equation is 1 MOLE of N2 + 3 MOLES of H2 yields

• 2 MOLES of NH3.

• Let’s look at our grilled cheese sandwich recipe again. What if I asked: If you start

• with 6 pieces of bread, and assume you have excess cheese (more cheese than youll need),

• how many grilled cheese sandwiches can you make? It’s tempting to just solve this in

• your head, but let’s solve it like we would a stoichiometry problem. We can use the recipe

• to find conversion factors. Remember conversion factors are ratios that equal 1.

• For every 2 pieces of bread, you need 1 piece of cheese. So you can write that as

• a conversion factor: 2 bread/ 1 cheese = 1.

• Similarly, for every 2 pieces of bread, you get 1 grilled cheese sandwich. That conversion

• factor looks like this: 2 bread/ 1 grilled cheese sandwich = 1

• And for every 1 piece of cheese, you get 1 grilled cheese sandwich. The conversion factor

• is: 1 cheese/ 1 grilled cheese sandwich = 1

• Back to our problem: Write down what you know, and what you want to get at the end,

• and leave a blank space for a conversion factor: 6 pieces of bread gives us…. some number

• of grilled cheese sandwiches. We want a conversion factor that has pieces

• of bread and # of grilled cheese sandwiches in it.

• Well use the conversion factor 2 bread/ 1 grilled cheese sandwich = 1. But careful

• - we want our units to cancel. So in this case, well invert our conversion factor.

• That’s the great thing about conversion factors - they equal 1, so you can flip them,

• and theyll still equal 1. That’s why we can use our conversion factor in either

• orientation.

• So we have 6 pieces of bread times (1 grilled cheese sandwich/ 2 pieces of bread)

• = some number of grilled cheese sandwiches. pieces of bread cancels...

• 6/2 = 3 grilled cheese sandwiches.

• I’m sure you could have done this problem in your head, but this is how youll want

• to write out all your stoichiometry problems. Always write out your work. That way, youll

• never accidentally divide instead of multiplying with a conversion factor - if you write it

• out you can see when your units cancel properly. And, if youre in a class, you might get

• partial credit for showing your work. It’s just a good habit.

• {{Example 1}} Now let’s try a stoichiometry problem

• using a balanced chemical equation. The balanced chemical equation will be just like our recipe.

• N2 + 3H2 → 2NH3 If you start with 10 molecules of N2, and

• you have an excess of H2, how many molecules of NH3 will you make?

• It’s a good idea to check that your equation is balanced. Count up the number

• of atoms on each side. On the left, we have 2 N and 6 H. And on the right we have

• 2 N and 6 H. Okay, it’s balanced. If your equation isn’t balanced, you have to balance it before

• you can do anything else, or stoichiometry won’t work. It would be like a bad recipe,

• like trying to make grilled cheese sandwiches and starting with one piece of bread and expecting

• 2 pieces of bread to magically show up in your finished sandwich. You can’t create

• bread and cheese out of nothing - and the same goes for your chemical reactions.

• Remember were going to use conversion factors. Start with what you know on the left,

• and what you want to wind up with on the right. 10 N2 times some conversion factor = some

• number of molecules of NH3 on the right.

• We get our conversion factors from our balanced chemical equation. The equation tells

• us for every 1 molecule of N2 you use 3 molecules of H2 to get 2 molecules of NH3. For this

• problem, we need a conversion factor with molecules of N2 and molecules of NH3. We can

• write it like this: 1 molecule of N2 over 2 molecules of NH3 = 1.

• Let’s put that into our problem. Remember we need to make sure the units cancel. We

• want to cancel N2 and wind up with NH3, so were going to flip our conversion factor.

• 10 N2 times 2NH3 / 1 N2 = the number of molecules of NH3

• N2 cancels. 10 times 2 = 20 molecules of NH3.

• In this stoichiometry problem, we went from molecules to molecules. It turns out,

• it’s the same kind of conversion as going from moles to moles. We could have asked - if

• you start with 10 moles of N2, how many moles of NH3 will you wind up with - and you’d

• get 20 moles of NH3. The Balanced chemical equation tells you the MOLE RATIOS of all

• the reactants and products in the chemical reaction.

• It gets a little more complicated when you start talking about mass - going from

• grams to grams - but the general idea is the same. It will take 3 steps: grams to moles,

• then moles to moles, and finally, moles to grams.

• Well tackle these types of stoichiometry problems in PART II.

Stoichiometry is a kind of calculation in chemistry. It’s sort of like bookkeeping.

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B1 cheese grilled cheese conversion grilled n2 bread

# Chemistry: Intro to Stoichiometry with Grilled Cheese Sandwiches | Homework Tutor

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林宜悉 posted on 2020/03/06
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