1. UNIT 11
STOICHIOMETRY
Stoichiometry is the study of quantitative relationships between the amounts of reactants used and amounts of products formed by a chemical reaction.

2. Stoichiometry is based on the law of conservation of mass.
The mass of reactants equals the mass of the products.

4. A mole ratio is a ratio between the numbers of moles of any two substances in a balanced equation.
The number of mole ratios that can be written for any equation is (n)(n – 1) where n is the number of species in the chemical reaction.

5. Balanced Equations and Mole Ratios
Coefficients in a BALANCED equation show the ratio of moles of reactants and products.
2H2 + O2  2H2O
3(3-1)
List all six mole ratios

6. Using Stoichiometry
All stoichiometric calculations begins with a balanced chemical equation.
4Fe(s) + 3O2(g) → 2Fe2O3(s)

7. Steps to solve mole-to-mole, mole-to-mass, and mass-to-mass stoichiometric problems
Complete Step 1 by writing the balanced chemical equation for the reaction.
To determine where to start your calculations, note the unit of the given substance.
If mass (in grams) of the given substance is the starting unit, begin your calculations with Step 2.
If amount (in moles) of the given substance is the starting unit, skip Step 2 and begin your calculations with Step 3.

8. 3. The end point of the calculation depends on the desired unit of the unknown substance.
If the answer must be in moles, stop after completing Step 3.
If the answer must be in grams, stop after completing Step 4.

10. Mole – Mole Calculations
Practice
Mole – Mole Calculations
Begin with mole
End with mole
One step!
N H2  NH3
How many moles of NH3 can be produced from 0.60 mol of N2?
Balance Equation
Determine mole ratio
Do conversion

11. Al + O2  Al2O3 How many moles of Al are needed for 3.7 mol Al2O3?
How many moles of O2 needed to react with 14.8 mol Al?

12. Why do reactions stop?
Reactions proceed until one of the reactants is used up and one is left in excess.
The limiting reactant limits the extent of the reaction and, thereby, determines the amount of product formed.
The excess reactants are all the leftover unused reactants.

13. Examples Building bicycles Making omelets
Need 2 wheels, 1 gear, 1 seat
Have 15 wheels, 6 gears, 4 seats
Making omelets
Need 3 eggs & 1 cup of cheese
Have 12 eggs & 10 cups of cheese

14. Determining Limiting Reactant
Perform conversion for each reactant = amount needed
Compare amount needed with amount present
Whichever doesn’t have enough = limiting

15. Na + Cl2  NaCl You have 6.7 mole of sodium and 3.20 mole of Chlorine
Which is limiting?
How much NaCl can be made?

16. Given 23 g of Fe and 10 g of O2 which is limiting?
Which is excess?
How much excess is there?
How much FeO can be produced?

17. Why do reactions stop?
Determining the limiting reactant is important because the amount of the product formed depends on this reactant.

18. Calculating the Product when a Reactant is Limiting
S8(l) + 4Cl2(g) → 4S2Cl2(l)
200.0g S and 100.0g Cl2
Determine which is the limiting reactant
mole of reactants: mol CI2; mol S8
mole ratios determine that for every 1 mol of S8, mol CI2 are available
mole ratio from equation is 4 mol Cl2 : 1 mol S8
Chlorine is limiting since there are less moles available than required by the equation.

19. Calculating the Product when a Reactant is Limiting
Calculating the amount of product formed
Multiply the amount of limiting reactant (Cl2) by the mole ratio relating S2Cl2 to Cl2.
190.4g S2Cl2 form

20. Calculating the Product when a Reactant is Limiting
Analyzing the excess reactant
Moles reacted
Multiply the moles of Cl2 used by the mole ratio relating S8 to Cl2.
mol S8
Mass reacted.
Multiply moles reacted by molar mass.
90.42g of S8
Excess remaining.
200.0g – 90.42g = g S8 in excess

21. Calculating the Product when a Reactant is Limiting
Using an excess reactant can speed up the reaction.
Using an excess reactant can drive a reaction to completion.

22. How much product?
Laboratory reactions do not always produce the calculated amount of products.
Reactants stick to containers.
Competing reactions form other products.

23. How much product?
The theoretical yield is the maximum amount of product that can be produced from a given amount of reactant.
The actual yield is the amount of product actually produced when the chemical reaction is carried out in an experiment.
The percent yield of a product is the ratio of the actual yield expressed as a percent.

24. CaCO3  CaO + CO2
28.4 g of CaCO3 produces 13.1 g of calcium oxide, what is the % yield
Actual yield =
Theoretical yield =
% yield =

25. CuCl + H2S  Cu2S + HCl 44.0 mol of CuCl forms 6.0 mol of Cu2S
Actual yield =
Theoretical yield =
% yield =