Stoichiometry

1
Stoichiometry

• Unit 7

2
Stoichiometry

• To make a smore, you need 2 graham crackers, 3
  squares of chocolate, and 1 marshmallow.
• If you have 20 graham crackers, 30 chocolate
  squares, and 10 marshmallows, how many smores can
  you make?

3
Stoichiometry

• What if you have 40 graham crackers, 30 chocolate
  squares, and 20 marshmallows?
• What if you have 18 graham crackers, 30 chocolate
  squares, and 10 marshmallows?

4
Stoichiometry

• Limiting Reagent
• Limits (determines) the amount of product that
  you can get
• Excess Reagent
• Reagent that there is more than enough of
• There is extra leftover when the reaction is
  complete

5
Stoichiometry

• Just like the smores, the same thing happens in
  chemical reactions.
• Example
• 2 H2 O2 ? 2H2O

H2 is L.R.
O2 is E.R.
4 molecules H2
3 molecules O2
6
Stoichiometry

• Solving Limiting Reagent Problems
• If 18.1 g NH3 is reacted with 90.4 g of CuO,
  which is the limiting reagent and how many grams
  of solid copper will be produced?

7
Stoichiometry

• Balance the equation.
• 2 NH3 3 CuO ? N2 3 Cu 3 H2O
• 2. Pick 1 of the reactants and 1 of the products.
  (Pick the product you are being asked about.)
  Using the given amount of reactant, figure out
  how much product you get. (3 steps)

8
Stoichiometry
1 mol NH3
63.55 g Cu
3 mol Cu
18.1 g NH3
2 mol NH3
1 mol Cu
17.04 g NH3

101 g Cu
9
Stoichiometry

• 3. Now take the other reactant, and repeat Step 2
  with the same product you used before. (3 steps)

1 mol CuO
63.55 g Cu
3 mol Cu
90.4 g CuO
3 mol CuO
1 mol Cu
79.55 g CuO

72.2 g Cu
10
Stoichiometry

• 4. Which reactant gave you less product?
• CuO
• 5. Thats the one that will run out first, so
  that is your limiting reactant.
• 6. Now, using your LIMITING REACTANT, go back and
  answer the original question.
• 72.2 g Cu

11
Stoichiometry

• Examples
• 2 H2 O2 ? 2 H2O
• Determine the limiting reactant in each case.

12
Stoichiometry

• a. 4.0 g H2 reacts with 3.0 g O2

1 mol H2
18.02 g H2O
2 mol H2O
4.0 g H2
2 mol H2
1 mol H2O
2.02 g H2

36 g H2O
1 mol O2
18.02 g H2O
2 mol H2O
3.0 g O2
1 mol O2
1 mol H2O
32.00 g O2

O2 is L.R.
3.4 g H2O
13
Stoichiometry

• b. 10.0 g H2 reacts with 10.0 g O2

1 mol H2
18.02 g H2O
2 mol H2O
10.0 g H2
2 mol H2
1 mol H2O
2.02 g H2

89.2 g H2O
1 mol O2
18.02 g H2O
2 mol H2O
10.0 g O2
1 mol O2
1 mol H2O
32.00 g O2
11.3 g H2O

O2 is L.R.
14
Stoichiometry

• c. 15.0 g H2 reacts with 10.0 g O2

1 mol H2
18.02 g H2O
2 mol H2O
15.0 g H2
2 mol H2
1 mol H2O
2.02 g H2

134 g H2O
1 mol O2
18.02 g H2O
2 mol H2O
10.0 g O2
1 mol O2
1 mol H2O
32.00 g O2

11.3 g H2O
O2 is L.R.
15
Stoichiometry

• d. 5.0 g H2 reacts with 30.0 g O2

1 mol H2
18.02 g H2O
2 mol H2O
5.0 g H2
2 mol H2
1 mol H2O
2.02 g H2

45 g H2O
1 mol O2
18.02 g H2O
2 mol H2O
30.0 g O2
1 mol O2
1 mol H2O
32.00 g O2

33.8 g H2O
O2 is L.R.