Stoichiometry
Moles, masses, reactions, oh my!

Stoikheion + metria

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

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From the Greek for “element” and
“the process of measuring”
Quantitative analysis of reactants and products
Moles
Molar masses
Balanced equations

Grinch Sandwiches

Ingredients per sandwich
◦ Three pieces of bread
◦ Sauerkraut (10 g)
◦ Toadstool (15 g)
◦ Arsenic sauce (5 g)

Balanced Equations

2H
2
+ O
2
 2H
2
0
◦ Coefficients = moles
◦ 2 moles of hydrogen react with 1 mole of oxygen to produce 2 moles of water
◦ What if you only had 1 mole of hydrogen?
◦ What if you had 10 moles of both hydrogen and oxygen?
◦ What if you needed to make 5 moles of water?

Law of Conservation of Mass




Convert each of the moles in the previous equation to grams
Add the reactants’ masses together.
Compare to the product’s mass.
Be in awe!

Mole Ratios


Equation must be balanced
Relate reactants to each other, products to each other, or reactants to products

Molar Masses

If mass is mentioned, find that substance’s molar mass

Making Stoichiometry Work

Dimensional Analysis
◦ Number, unit, substance…always!

Problem #1

When 2.50 g of hydrogen gas are mixed with an excess of oxygen gas, how many grams of water should be produced?

Problem #1—step 1

Write a balanced equation.
◦ 2H
2
+ O
2
 2H
2
0

Problem #1—step 2


Begin with your given information and make a plan of conversions.
◦ Grams of hydrogen  moles of hydrogen  moles of water  grams of water
Write it as dimensional analysis.
g H
2 x mol H
2 g H
2 x mol H
2
O x g H
2
O = mol H
2 mol H
2
0

Problem #1—step 3

Fill in your numbers.
2.5 g H
2 x 1 mol H
2
18.02 g H
1 mol H
2
2
0
2.02 g H
2
O = x 2 mol H
2
O x
2 mol H
2
• Cross out what divides out.
• Calculate your answer.

Problem #1—step 4
2.5 g H
2 x 1 mol H
2
2.02 g H
2 x 2 mol H
2
O x
2 mol H
2
18.02g H
2
O = 22.3 g H
2
O
1 mol H
2
0
• How much oxygen gas would be required?

Problem #2

The thermite reaction: aluminum foil reacts with ferric oxide

Problem #2

Wow! Now let’s assume that 0.010 g of aluminum reacted with the excess ferric oxide on the ball bearing.
What mass of aluminum oxide should be produced?

Problem #2—Step 1

Write a balanced equation.
2Al + Fe
2
O
3
 Al
2
O
3
+ 2Fe

Problem #2—Step 2

Write your plan.
g Al x mol Al x mol Al
2
O g Al
3 x g Al
2
O mol Al mol Al
3
2
O
3
=

Problem #2—Step 3

Place the proper numbers and calculate.
0.010 g Al x 1 mol Al x 1 mol Al
2
O
26.98 g Al 2 mol Al
3
101.96 g Al
1 mol Al
2
O
2
3
O
3
= x

Problem #2—Answer

0.0189 g of Al
2
O
3 should be formed

Problem #3

What if only 0.0150 g of Al
2
O
3 were actually produced in the previous problem? What is the percent yield?

So, you can calculate the % yield
◦ % yield = actual amt x 100 theoretical amt

Problem #3

Thus, the % yield calculation
0.0150 g x 100 = 79.4 % yield
0.0189 g

Limiting Reagent
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Ideally, the perfect amount of each reactant
Often one reactant limits how much product can be formed—limiting reagent (LR)
Often the other reactant(s) is(are) in excess (ER)
You will be given at least 2 quantities of reactants

Grinch Sandwiches

Ingredients per sandwich
◦ Three pieces of bread
◦ Sauerkraut (10 g)
◦ Toadstool (15 g)
◦ Arsenic sauce (5 g)

Problem #4

0.500 g of silver nitrate in solution reacts with 0.750 g of tin (IV) chloride in solution. If 0.400 g of solid silver chloride are retrieved, what is the percent yield?

Problem #4—Step 1

Write a balanced equation.
4AgNO
3
A
+ SnCl
4
B
 4AgCl + Sn(NO
3
C D
)
4

Write shorthand, to save time.
You’ll write the real stuff in the answer.

Problem #4—Step 2
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
Ask yourself, when I have 0.500 g of
AgNO
3
, how many grams of SnCl
4
I really need? This will determine do the LR.
Set up the plan g A x mol A x mol B x g B = g A mol A mol B

Problem #4—Step 3

Plug in the numbers.
0.5 g A x 1mol A x 1 mol B
169.88g A 4 mol A x 260.50 g B = 0.192 g SnCl
4
1 mol A

Problem #4—Step 4

Analyze and determine the LR
◦ 0.750 g of SnCl
4 are available, and only
0.192 g of it need to be used to completely react with the AgNO
3
• Which substance will be in excess?
• Which substance will you run out of first?

Problem #4—Step 5
• Now, use the given amount of the
LR to find how much product you should be able to make.
• Set up your plan.
g A x mol A x mol C x g C = g A mol A mol C

Problem #4—Step 6
• Plug in your numbers and calculate.
0.5 g A x 1 mol A x 4 mol C x
169.88 g A 4 mol A
143.32 g C = 0.422 g AgCl should be
1 mol C formed

Problem #4—Step 7
• Look at the actual amount of product that was formed, and determine percent yield using the theoretical you just calculated.
0.400 g AgCl x 100 = 94.8% yield
0.422 g AgCl

Practice
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Sit down with some bread, sauerkraut, toadstools, and arsenic sauce, and work some stoichiometry problems just for fun.