
Stoichiometry
 What exactly IS stoichiometry?
1.
A balanced chemical equation is very much like a recipe: it tells you the ingredients needed, how
much is needed of each ingredient, as well as the proportions of the ingredients in case the “recipe” needs to
be adjusted. Consider the following reaction, as well as the different ways that we can interpret it:
2 H2
+
O2
→
2 H2O
 On the particle level:
+
→
 On the mole level:
 On the mass (gram) level:
x NO:
 YES:
2.
The most important piece of information we can take from a balanced equation is the
MOLE RATIO. IT IS THE HEART AND SOUL OF EVERY STOICHIOMETRY PROBLEM!
N2 + 3 H2 → 2 NH3
 What is the mole ratio of nitrogen to hydrogen? (N2:H2) ____________________
 What is the mole ratio of hydrogen to ammonia? (H2:NH3) ____________________
 Using the correct mole ratio from above, solve this problem:
How many moles of hydrogen are needed to prepare 312 moles of ammonia?
*This is called a MOLE-MOLE problem in stoichiometry!
3.
Remember Molar Mass??? PERIODIC TABLE TIME!
Calculate these Molar Masses TO THE TENTH! And don’t forget….g/mol!
a) NaOH
b) HCl
c) SrS
d) CuBr2
e) Al(NO3)3
f) C6H12O6
 Of course, more useful to us is information in grams, so we will need to rely on our
good buddy Molar Mass to get us in and out of Moleville from Mass City!
4.
NOW, with a combination of the mole ratio and molar mass, we can solve the most
important stoichiometry problem…the MASS-MASS problem!!!
Ca + 2 HCl → CaCl2 + H2
 How many grams of CaCl2 can be produced from 127.5 grams of HCl? 2 ways:
 Now you try one, choosing your favorite way to solve it!
2 Fe + CuBr2 → Cu + 2 FeBr
 How many grams of Cu can be produced from 250. grams of Fe?
5.
What about Liter Land and Particle Town on our Maps to Moleville???
Remember, our good buddy the mole has multiple meanings: molar mass (as we’ve
just seen), 6.02 x 1023 particles (no duh!), and . . . . . 22.4 Liters! We’ll do some more
with this during our study of gases next unit! Let’s go back to our ammonia reaction:
N2 + 3 H2 → 2 NH3
o How many liters of NH3 gas can be produced from 122.0 moles of N2 gas at STP?
o How many molecules of NH3 can be produced from 175 moles of H2?
6.
In chemical equations, the reactants can be viewed as one of these:
 limiting reactant:
 excess reactant:
 If you were going to bake a cake and you had the following ingredients available,
which would be the limiting reactant?
10 pounds of flour, 50 pounds of sugar, 5 gallons of vanilla, 20 gallons of milk, 1 egg
Limiting reactant: _______________
 Once we know the limiting reactant, we can calculate the theoretical yield for each
reaction, and then once we run a reaction, we can see how well it went by
comparing our actual yield to the theoretical yield: percentage yield!
Limiting Reactant
Sample Problem
Two
Mass-Mass
Problems!
Percent Yield
Formula
 Your turn!!! Determine the limiting reactant, the theoretical yield, AND the percentage
yield if 16.1 g of Br2 are mixed with 8.42 g of Cl2 and 21.1 g of BrCl actually form.
Two
Mass-Mass
Problems!
Br2
+ Cl2 → 2 BrCl
Limiting Reactant: ___________________
Theoretical Yield: _____________________
Percent Yield
Calculation
Percentage Yield: ____________________________