1. The study of quantities of substances in chemical reactions
Stoichiometry
The study of quantities of substances in chemical reactions

2. Interpreting Chemical Equations N2 + 3 H2  2 NH3
Particles:
1 molecule of Nitrogen reacts with 3 molecules of Hydrogen to produce 2 molecules of Ammonia (NH3).
Moles:
1 mole of Nitrogen reacts with 3 moles of Hydrogen to produce 2 moles of Ammonia (NH3).
The important thing to notice is that the coefficients determine the ratio of each reactant to each product.

3. Interpreting Chemical Equations N2 + 3 H2  2 NH3
Mass
Balanced reactions must obey the law of conservation of mass.
Using the mole idea you can determine the mass of species in this equation:
used
1 mole N2 = 28 g
3 mole H2 = 6 g
produced
2 moles NH3 =
34g
Volume
@STP conditions
22.4 L of N2 reacts with
67.2 L of H2 to produce
44.8L of NH3
Notice that volume of gases is NOT conserved
≠ 44.8

4. Which of the following are conserved in a chemical reaction?
Mass
The rearrangement of atoms during the reaction can change the # of everything else. But mass is conserved.
Volume of Gas
Moles
Mass
# of Particles

5. Mole-Mole Calculations N2 + 3 H2  2 NH3
Relating moles of reactants to moles of products.
We do not always use full moles of reactants, or want full moles of products.
We more often deal with partial moles.
If this reaction started with 2 moles of N2, how many moles of NH3 could be produced?
1: :4
It’s a simple ratio
For more complicated ratios use this guide

6. Mole-Mole Calculations N2 + 3 H2  2 NH3
So if we start with .75 moles of N2, how much ammonia is produced?
How many moles of H2 must be used?
If we want to produce 3.18 moles of ammonia, how many moles of hydrogen do we need to use?
Known moles x
Coefficient of what you want to find out
Coefficient of what you know
Mole conversions practice.

7. 4Al + 3 O2  2 Al2O3 Write three mole ratios from this equation
How many moles of Aluminum metal are needed to produce 3.7 moles of Aluminum Oxide?
How many moles of O2 are required to react completely with 14.8 moles of Al?
Calculate the moles of Al2O3 formed when 0.78n of O2 react with Al.

8. Mass-Mass Calculations N2 + 3 H2  2 NH3
Our tools do not directly measure moles.
The balances measure…
Mass (g)
So we often must convert to moles from mass, and mass from moles.
Solving mass-mass problems:
Convert the given quantity to MOLES.
(Divide mass by molar mass)
Calculate unknown moles by multiplying by a mole ratio. (unknown/known)
Convert the answer in # 2 to mass.
(Multiply by molar mass)

9. Mass-Mass Calculations N2 + 3 H2  2 NH3
If 8.5 grams of Nitrogen are reacted, what mass of ammonia can be produced?
If 16.4 grams of Hydrogen are reacted, how many grams of Nitrogen are needed to fully react?
Solving mass-mass problems:
Convert the given quantity to MOLES.
(Divide mass by molar mass)
Calculate unknown moles by multiplying by a mole ratio. (unknown/known)
Convert the answer in # 2 to mass.
(Multiply by molar mass)

10. K2O + H2O  2 KOH
Calculate the mass of KOH produced when 4.80 g of K2O is reacted.
How many grams of water are needed for this reaction?
If 50.0 g of KOH is produced, what is the mass of Water used?
Solving mass-mass problems:
Convert the given quantity to MOLES.
(Divide mass by gfm)
Calculate unknown moles by multiplying by a mole ratio. (unknown/known)
Convert the answer in # 2 to mass.
(Multiply by gfm)

11. Warm-Up
2 Al + 3 Cl2  2 AlCl3
If 70.9 g of Chlorine Gas are used, how many moles of Aluminum Chloride can be produced?
Solving mass-mass problems:
Convert the given quantity to MOLES.
(Divide mass by molar mass)
Calculate unknown moles by multiplying by a mole ratio. (unknown/known)
3. Convert the answer in # 2 to mass.
(Multiply by molar mass)
What is the mass of the AlCl3 ?

12. Other legs of the mole wheel
If given a mass of a reactant
you can find # of particles produced
If given the volume of a reactant
you can find the mass produced
If given the # of molecules of a product
you can find # molecules of reactant

13. 2 H2O2  2 H2O + O2 Starting with 29.2 g of hydrogen peroxide
MnO2
2 H2O2  2 H2O + O2
Starting with 29.2 g of hydrogen peroxide
How many molecules of Oxygen are produced?
What is the volume of the Oxygen gas produced? (assume STP conditions)
What mass of water is produced?

14. 2 SO2 + O2 2 SO3
How many liters of Oxygen are needed to produce 24g of Sulfur Trioxide? (All species are in the gas STP)

15. Limiting Reagent analogy
IF you have:
40 slices of turkey
28 slices of cheese
6 heads of lettuce
1 mole of tomatoes
Jars of mayo and mustard
2 slices of bread
How many sandwiches can you make?
What limits your production of sandwiches?
Bread is your “limiting reagent”

16. Limiting Reagent analogy
Your reaction is your recipe
Seldom do you have perfect amounts of each ingredient.
The limiting reagent (reactant) is what determines the amount that is produced.
The other ingredients are considered to be excess reagents
Cheese, turkey, tomatoes…

17. Limiting Reagent Definition
Limiting Reactant: (a.k.a. Limiting Reagent) The substance that controls the quantity that can form In a chemical reaction. Excess Reactant: (a.k.a. excess reagent) The substance that is not used up completely in a reaction.

18. Determining Limiting Reagent 2 Na + Cl2  2 NaCl
Starting with 6.7n of Na, and 3.2n of Cl2
How many moles of NaCl can be produced?
A. Determine limiting reagent
B. Use limiting reagent to determine amount of product formed.
Start with a known amount of one reactant.
Use the mole ratio to determine the amount of product.
Determine how much the other reactant would produce.
The reactant producing the LEAST is limiting.

19. 2 Na + Cl2  2 NaCl Starting with 6. 70n of Na, and 3
2 Na + Cl2  2 NaCl Starting with 6.70n of Na, and 3.20n of Cl How many moles of NaCl can be produced?
If 6.7n Na are used
6.7 x ½ = 3.35n Cl2 needed
Compare the 3.35n Cl2 needed,
to 3.20n Cl2 you have.
We have less Cl2 than we need so Chlorine is the limiting reagent.
Use the limiting reagent to determine the moles of NaCl produced.
Start with a known amount of one reactant.
Use the mole ratio to determine the amount of product.
Determine how much the other reactant would produce.
The reactant producing the LEAST is limiting.

20. 2 Na + Cl2  2 NaCl Starting with 6. 70n of Na, and 3
2 Na + Cl2  2 NaCl Starting with 6.70n of Na, and 3.20n of Cl How many moles of NaCl can be produced?
We have determined the 3.2n of Cl2 to be limiting.
Now it becomes a regular mole-mole problem.
3.2n Cl2 x 2/1 =
6.4n NaCl
You could then find the mass of NaCl produced.

21. What determines the amount of product?
The limiting reagent!

22. 16 Cu + S8 8 Cu2S Starting with 1.25n Cu and .78n of S
Mass
Moles
Starting with 1.25n Cu and .78n of S
determine the moles of Copper (I) Sulfide made.
What mass of Cu2S is made?
0.625 n of Cu2S
99.4 g of Copper sulfide

23. Find the volume of this Hydrogen @ STP
Mg HCl  MgCl2 + H2
Mass
moles
How many grams of Hydrogen can be produced when 6.00 g of HCl is reacted with 5.00 g of Mg?
Convert each reactant to moles.
Determine limiting reagent by multiplying by the mole ratio.
Use limiting reagent to determine moles of product.
Multiply moles of product by GFM to find mass
Find the volume of this Hydrogen @ STP

24. 2 H2 + O2  2 H2O
Mass
Moles
Given 40grams of Hydrogen and 90 grams of Oxygen, determine the limiting reagent.
Determine the mass of water that can be produced.
How much excess reagent remains?

25. Percent Yield Why don’t you always get 100% on your chemistry tests?
There are many possible reasons
You went skiing during the important lectures.
You worked on your AP history homework instead of studying for chem.
Rippet is not as good as he thinks he is.

26. % Yield
In Chemical Reactions we don’t always get 100% of the products our equations predict.
Remember the Magnesium Burning Lab?
2 Mg + O2  2 MgO
Theoretically all the Mg should have converted to MgO.
In reality, some things went wrong.

27. % Yield An equation predicts the 2 Mg + O2  2 MgO THEORETICAL YIELD
When a reaction is carried out in the lab we get
ACTUAL YIELD
2 Mg + O2  2 MgO
We reacted 2.5g of Mg, what mass of MgO could be produced?
Assume there is unlimited O2 from the air so Mg is the limiting reagent.
0.103 n of Mg x 2/2 = 0.103n MgO
We only had 3.61 grams produced…
So we do NOT have 100% yield!

28. Multiply by 100, then add the “%” sign
% Yield
Actual (lab results)
Theoretical (equation prediction) X
100 %
Multiply by 100, then add the “%” sign
3.61 / 4.15 = .869
.869 x 100 +”%”
= 86.9% Yield

29. CaCO3  CaO + CO2
What is the % Yield if 24.8 g of CaCO3 is heated to give 13.1g of CaO?
Convert g CaCO3 to moles of CaCO3 .
Only 1 reactant, so it’s the limiting reagent.
Mole ratio is 1:1
Convert moles of CaO to grams CaO.
Use % Yield Formula: actual x 100%
theoretical

30. 2 Al + 3 CuSO4  Al2(SO4)3 + 3 Cu
What is the % yield when 4.65g of Copper is made when 1.87g of Al reacts with excess Copper Sulfate.
Excess means there is plenty, the other reagent is limiting.
Convert grams Al to moles Al.
Find moles of Cu expected (multiply by mole ratio)
Convert moles Cu to grams Cu.
This is the Theoretical amount of Copper
Use the % Yield Formula.

31. Which balloon will have the greatest volume of gas
Which balloon will have the greatest volume of gas? NaHCO3 + HC2H3O2  NaC2H3O2 + CO2 + H2O
Reaction
Mass of NaHCO3
Moles of Acetic Acid 1
0.50g
0.02n 2
1.00g 3
1.50g 4
2.00g 5
3.00g