Stoichiometry allows us to use balanced chemical equations to determine the amounts of reactants and products involved in chemical reactions. It treats the chemical equation like a recipe, using mole ratios derived from the coefficients to solve mole-mole, mole-mass, and mass-mass problems. For example, if 2 moles of hydrogen gas react with 1 mole of oxygen gas to produce 2 moles of water, how many moles of oxygen are needed to produce 4 moles of water? By using the 1:1 mole ratio of oxygen to water given in the balanced equation, we can determine that 4 moles of oxygen are needed.

1. Stoichiometry

3. What is Stoichiometry
• Using a balanced chemical equation to
determine how much reactant or product is
consumed or produced in a chemical reaction.
• 2 H2 (g) + O2 (g)
2 H2O (l)
– How many moles of H2O are produced from the
reaction of 2 moles of H2?
– How many moles of O2 are required to produce 4
moles of H2O?

4. Chemical Equations
• 2 H2 (g) + O2 (g)
H2O (l)
4 moles molecules + 2 moles molecules
4 moles molecules
Coefficients in Chemical Equation represents mole ratios
http://www.chemistry.ohio-state.edu/betha/nealChemBal/

5. Stoichiometry
“Chemical Equation = recipe”
• 2 bread slices + 1 cheese slice
1 sandwich
– Suppose need to make as many cheese sandwiches as possible
for a party and have 20 slices of bread...how many slices of
cheese are needed?
• By inspection of recipe (chemical equation)
• Or use coefficient ratio (mole ratio) from equation:
•
– 2 bread slices = 1 cheese slice
1cheese _ slice
cheese slices = 20 bread slices x
2bread _ slices
= 10 cheese slices needed
Image source: http://www.fotosearch.com/clip-art/sandwich.html

7. Mole – mole problem

8. Molar Mass Review

9. Mole-Mass problems

10. Mass-Mass Problems

11. Try the following:
How many moles of Fe2O3 will be produced from
the reaction of 1.50 mol of iron with Oxygen?
a) Write the balanced equation
– 4 Fe + 3 O2
2 Fe2O3
b) Solve
– ….mol Fe2O3 = 1.50 mol Fe x
= 0.75 mol Fe2O3
2 molFe
2O 3
4 molFe

12. Using grams:
Consider the unbalanced reaction:
Al (s) + Fe2O3 (s)
Al2O3 (s) + Fe (l)
If you react 4.0 g of Al with excess Fe2O3, how many moles of Fe
will be produced??
• First balance the equation:
2 Al (s) + Fe2O3 (s)
Al2O3 (s) + 2 Fe (l)
• Chemical equation speaks to us in moles, not grams! So to use
the mole ratio it gives us (2mol Al: 2 mol Fe),
– first need to convert 4.0 g Al to moles of Al
– Then take moles of Al and “convert” to moles of Fe using mole ratio
from balanced equation (coefficient ratio) as was done on previous
slides.

13. Molar Mass of Al is 26.98 g
Molar Mass of Fe is 55.85 g
4.0 g Al
1 mole Al
2 mole Fe
55.85 g Fe
26.8 g Al
2 mole Al
1 mole Fe
= 8.3 g Fe