1. Stoichiometry
“In solving a problem of this sort, the grand thing is to be able to reason backward. This is a very useful accomplishment, and a very easy one, but people do not practice it much.”
Sherlock Holmes, in Sir Arthur Conan Doyle’s A Study in Scarlet

2. Review: Chemical Equations
Chemical change involves a reorganization of
the atoms in one or more substances.
C2H5OH + 3O2 ® 2CO H2O
reactants
products
When the equation is balanced it has quantitative significance:
1 mole of ethanol reacts with 3 moles of oxygen to produce 2 moles of carbon dioxide and 3 moles of water

3. Calculating Masses of Reactants and Products
Balance the equation.
Convert mass or volume to moles, if necessary.
Set up mole ratios.
Use mole ratios to calculate moles of desired substituent.
Convert moles to mass or volume, if necessary.

4. Working a Stoichiometry Problem
6.50 grams of aluminum reacts with an excess of oxygen. How many grams of aluminum oxide are formed.
1. Identify reactants and products and write the balanced equation. 4 Al + 3 O2 2
Al2O3
a. Every reaction needs a yield sign!
b. What are the reactants?
c. What are the products?
d. What are the balanced coefficients?

5. Working a Stoichiometry Problem
6.50 grams of aluminum reacts with an excess of oxygen. How many grams of aluminum oxide are formed?
4 Al O2  2Al2O3
6.50 x 2 x ÷ ÷ 4 =
12.3 g Al2O3
6.50 g Al
1 mol Al
2 mol Al2O3
g Al2O3 =
? g Al2O3
26.98 g Al
4 mol Al
1 mol Al2O3

6. Limiting Reactant The limiting reactant is the reactant
that is consumed first, limiting the amounts of products formed.

7. Consider the following reaction that occurs in a fuel cell: 2H2 + O2 → 2H2O This reaction, properly done, produces energy in the form of electricity and water. Suppose a fuel cell is set up with 150 g hydrogen gas and 1500 g of oxygen gas. How many grams of water can be formed?

8. Adipic acid, H2C6H8O4, is used to produce nylon
Adipic acid, H2C6H8O4, is used to produce nylon. The acid is made commercially by a controlled reaction between cyclohexane and oxygen: 2C6H12 + 5O2→ 2 H2C6H8O4 + 2H2O a. Assume that you carry out this reaction with 25.0 g cyclohexane and that it is the limiting reactant. What is the theoretical yield of adipic acid?

9. b. If you obtain 33.5 g of adipic acid from your reaction, what is the percent yield of adipic acid?

10. Example 6 Consider the following reaction: P4(s) + 6F2(g)  4PF3(g)
What mass of P4 is needed to produce 85.0 g of PF3 if the reaction has a 64.9% yield?
46.1 g P4
64.9% = (85.0 g PF3 / x)(100); x = g PF3
( g PF3)(1 mol PF3 / g PF3)(1 mol P4 / 4 mol PF3)( g P4 / 1 mol P4) = 46.1 g of P4 needed