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Determine mole ratios from balanced chemical equations
Apply mole ratios appropriately as conversion factors to perform mole-mole, mass-mole, and mass-mass calculations
Determine the limiting reactant and excess reactant for a given reaction
Calculate the amount of excess reactant remaining when a reaction is complete
Explain the concepts of actual, theoretical, and percent yield
Compute theoretical yield, actual yield, and percent yield

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Stoichiometry is the calculation of quantitative(measurable) relationships of the reactants and products in chemical reactions
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Or to put it more simply, if the amount of one substance of a chemical reaction is known, the amount of any other substance in that reaction can be calculated
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For example, in the Haber process, which is used to produce ammonia:
3 H
2
(g) + N
2
(g) → 2 NH
3
(g)
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If the amount of hydrogen is known, then we can find the amount of ammonia that can be produced, as well as the amount of nitrogen needed to completely react with the hydrogen

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This is analogous to adjusting a recipe to make a certain amount of cookies
Macadamia Nut Cookies - Makes 4 Dozen
1/2 cup butter 3/4 cup white sugar
1 egg 1 teaspoon vanilla extract
1 1/4 cups all-purpose flour 1/2 teaspoon baking soda
1/2 teaspoon salt 8 ounces white chocolate
1 (6.5 ounce) jar macadamia nuts, chopped

A.
B.
C.
D.
E.
3
4
1
2
5

A.
B.
C.
D.
E.
3
4
1
2
5

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It works similarly in chemistry, but stoichiometry is built around the central concept of the mole ratio
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Mole ratios, as previously seen, are used to convert moles of one substance to moles of another substance
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They were previously found using the chemical formula of a compound
Cu
2
SO
4
1 mole CuSO
4
: 2 moles Cu : 1 mole S : 4 moles O

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So this mole ratio could be used, for example, to convert a given number of moles of copper into moles of sulfur and/or oxygen
Cu
2
SO
4
→ 1 mole CuSO
4
: 2 moles Cu : 1 mole S : 4 moles O
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In the space below, give the mole ratio that would be used to convert: a) moles Cu to moles S and b) moles O to moles S

A.
B.
C.
D.
7.5 x 10 -3
1.5 x 10 -2
3.0 x 10 -2
4.5 x 10 -2

A.
B.
C.
D.
.22 moles
.44 moles
.11 moles
72 moles

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All of these ideas function identically when dealing with a chemical equation instead of a single chemical compound
The only difference is the mole ratio is derived from the coefficients of a balanced equation instead of using the subscripts in a formula
So just like before:
1. Start by identifying the given and the unknown
2. If the given and unknown are different substances, a mole ratio must be used
3. The given’s unit must be moles in order to use a mole ratio

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So first let’s make sure we know how to get a mole ratio from a balanced chemical equation:
N
2
(g) + 3 H
2
(g) → 2 NH
3
(g)
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The coefficients give the ratios of the substances involved in the reaction
For every __________ of N
2 used, __________ of H
2 will be used.
For every __________ of H
2 used, __________ of NH
3 will be made.
For every __________ of NH
3 formed, you need ________ of N
2
.

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Now take these ratios and write them as conversion factors:
N
2
(g) + 3 H
2
(g) → 2 NH
3
(g)

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And lets try a little sample exercise action, shall we?
N
2
(g) + 3 H
2
(g) → 2 NH
3
(g)
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If there are 6.882 x 10 -1 moles of nitrogen, how many moles of hydrogen are needed to react completely with the nitrogen?

A.
B.
C.
D.
2.25 x 10 3 moles
4.50 x 10 3 moles
9.00 x 10 3 moles
1.00 moles

A.
B.
C.
D.
415 moles
104 moles
.121 moles
.0304 moles

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Let’s take a moment to talk about ham sandwiches
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To make a good ham sandwich, a person needs two pieces of bread and 5 pieces of ham
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If this “recipe” is thought of as being a chemical reaction, what would it look like?

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With that recipe in mind, how many sandwiches could be made if you had 11 pieces of bread and more ham than you could ever possibly use(excess ham)?

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With that recipe in mind, how many sandwiches could be made if you had 40 pieces of ham and more bread than you could ever possibly use (excess bread)?

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Now if we have 11 pieces of bread and 40 pieces of ham, how many sandwiches can we make?

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Bread is what we call the limiting reactant because it is the reactant that limits how much product can be made
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It can also be thought of as the reactant that is used up first, but it is never actually completely used up…just very, very close
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Ham is the excess reactant because it is the reactant of which a significant amount remains at the end of the reaction
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It can also be thought of as the reactant that is not used up in a reaction

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Sticking with our 40 pieces of ham and 11 pieces of bread recipe
2 + 5
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Anytime the amount of both reactants is given, you have to determine which reactant is the limiting reactant
There are two ways to do this:
1. Determine the amount of product that can be formed from each reactant; the limiting reactant will always produce the smaller amount
2. Determine if you have enough of one reactant to go with the other; do you have enough ham to use all 11 pieces of bread?

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The number of sandwiches from each ingredient way:
2 + 5

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The do I have enough of the other ingredient way:
2 + 5

2.0 mol of HF react with 4.5 moles of SiO
2
Which is the excess reactant?
. Which reactant is limiting?
SiO
2
(s) + 4 HF(g) → SiF
4
(g) + 2 H
2
O(l)

If 20.5 g of chlorine reacts with 20.5 g sodium, which reactant is limiting? Which is the excess reactant?

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To recap what we know about limiting reactants:
1. If the amount of both reactants is known, you have to figure out which is the limiting reactant
2. The limiting reactant can be identified by calculating the amount of product each would form; the limiting reactant always produces a smaller amount of product
3. The limiting reactant be identified by calculating the amount of one reactant needed to completely use up the other, and comparing this to the amount actually present
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Additionally, once the limiting reactant is known, it becomes the given for all further calculations

How many moles of the excess reactant are left over when 0.750 moles of N
2
H
4 is mixed with 0.500 moles of H
2
O
2
?
N
2
H
4
(l) + 2 H
2
O
2
(l) → N
2
(g) + 4 H
2
O(g)

How many moles of nitrogen are produced when 0.750 moles of N
2 is mixed with 0.500 moles of H
2
O
2
?
H
4
N
2
H
4
(l) + 2 H
2
O
2
(l) → N
2
(g) + 4 H
2
O(g)

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So these are the things you should be able to determine from this section:
1) the limiting reactant
◦ Must be found when the amount of both reactants is known
◦ The limiting reactant is the given for every other calculation once it has been found
2) the excess reactant
3) the amount of excess reactant remaining(in grams or moles)
4) the amount of product formed(based on the limiting reactant)
◦ The amount of product is always based on the limiting reactant

A.
B.
C.
Ag
S
8
Ag
2
S
16 Ag(s) + S
8
(s) → 8 Ag
2
S(s)

C.
D.
A.
B.
15.5 g
2.30 g
1.15 g
23.0 g
16 Ag(s) + S
8
(s) → 8 Ag
2
S(s)

C.
D.
A.
B.
.297 g S
8
13.45 g Ag
16 Ag(s) + S
8
(s) → 8 Ag
2
S(s)
11.45 g Ag
1.703 g S
8

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Percent yield is used to measure how effective a reaction is
For example, using our sandwich recipe:
2 bread + 5 ham → 1 sandwich
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How many sandwiches can we make from 11 pieces of bread and
40 pieces of ham?
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This is the MAXIMUM amount of sandwiches that can be made, or the theoretical yield of sandwiches

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The theoretical yield is what you find any time you use stoichiometry to calculated the amount of product formed by a reaction
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So this means there are TWO things you need to know about the theoretical yield!
1. The theoretical yield is the amount of product calculated using stoichiometry
2. The theoretical yield is the maximum POSSIBLE amount of product a reaction CAN produce

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So if everything goes flawlessly with a reaction and the reactants are used up as much as possible, you will obtain the theoretical yield from a chemical reaction; sadly, this will never happen
The theoretical yield of sandwiches was 5 sandwiches, but lots of things can go when you’re making sandwiches, as I’m sure you know
How many sandwiches would we get if you dropped three pieces of bread on the ground and couldn’t use them?
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This would be the amount of sandwiches you actually were able to make, and this is referred to as the actual yield

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The percent yield measures the efficiency of a reaction, or in this case, a sandwich making session
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The percent yield indicates what percent of the theoretical yield was actually obtained

A.
B.
C.
D.
100%
104%
95.5%
54.7%

C
6
H
6
(l) + Cl
2
(g)  C
6
H
5
Cl (s) + HCl (g)
A.
B.
C.
D.
73.17%
137%
94.8%
69.4%