1. Stoichiometry
Unit 4 (Ch 9)

2. What is it?
Study of quantitative relationships between amounts of reactants used and products formed by a chemical reaction
Based on the law of conservation of mass which states that mass cannot be created or destroyed, but it can change forms.

3. 2 slices of bread + 1 slice of cheese  1 grill cheese sandwich
Recipe
2 slices of bread + 1 slice of cheese  1 grill cheese sandwich
Why is it important to know the recipe?
To determine how much is needed or how much will be produced
How many sandwiches can you make if you have 20 slices of bread and LOTS of cheese?

4. Interpreting the Equations
When looking at a balanced equation, the coefficients represent the 4 different possible relationships that the reactants and products can have
Particles
Moles
Mass
Volume (if only gases are present!!)

5. 4 Different Relationships 2 H2(g) + O2 (g)  2H20 (g)
1. Particles
What are the three different types?
Atoms
Molecules
Formula units
What are the relationships of the particles in the chemical equation?
For every 2 molecules of hydrogen that react with 1 molecule of oxygen, 2 molecules of water are produced
Are there equal numbers of each element on both sides?
What law does it obey?
Conservation of MATTER

6. 2 H2(g) + O2 (g)  2H20 (g)
2. Moles
What are the mole relationships in the chemical reaction?
For every 2 moles of hydrogen that react with 1 mole of oxygen, 2 moles of water are produced
Are the number of moles equal?
No, but it indirectly obeys the law of conservation of mass

7. Mole Ratios = Conversion Factors
What is it?
A ratio between the numbers of moles of any two substances in a balanced chemical equation
2H2 + O2  2H2O
What are the 6 different mole ratios that exist?
Hint: 2 mol H mol O2
1 mol O mol H2

8. 2 H2(g) + O2 (g)  2H2O (g) 3. Mass Must obey the conservation of mass
Previously, we stated that if 2 moles of hydrogen (_____g) reacts with 1 mole of oxygen (_____g), then 2 moles of water are produced.
1 mole of H2 = 2.00 grams of H2
1 mole of O2 = grams of O2
What is the mass of 2 moles of water?
1 mole of H2O = grams of H2O

9. 2 H2(g) + O2 (g)  2H2O (g) Pressure at 1 atm,760 mm Hg or 101.3 KPa
4. Volume (liters)
Must be at STP
Pressure at 1 atm,760 mm Hg or KPa
Temperature at 0 degrees Celsius or 273K
All reactants and products must be gases
The equation tells you the volume relationship between the reactants and products

10. 2 H2(g) + O2 (g)  2H2O (g)
What are the volume relationships in the chemical reaction?
For every 2 liters of hydrogen that reacts with 1 liter of oxygen, 2 liters of water are formed
Is the volume conserved?

11. 2 K(s) + 2 H20(l) 2KOH(aq) + H2(g)
Interpret the above equation in terms of relative numbers of representative particles, numbers of moles, and masses of reactants and products.
Mole relationship
2 mol K + 2 mol H2O 2 mol KOH + 1 mol H2

12. 2 K(s) + 2 H20(l) 2KOH(aq) + H2(g)
Particles (Be specific!! Three types)
2 atoms K + 2 molecules H20 2 form. units KOH + 1 molecule H2
Masses
____ g K + _____g H20 ______g KOH + _____g H2
1 mole K = ______ g K
1 mole H2O = _________g H20
1 mole KOH = _______g KOH
1 mole H2 = _________g H2

13. Mole to Mole Calculations
N2 (g) + 3H2 (g)  2NH3 (g)
What are the mole ratios for this chemical equation?
Hint: there are 6 of them

14. N2 (g) + 3H2 (g)  2NH3 (g)
How many moles of nitrogen must react to produce 5.5 moles of ammonia?
Setup:
5.5 mol NH3 = _______ mol N2
Conversion Factors and/or Mole ratios:
2 mol NH3 = 1 mol N2
Work:

15. Moles of A to Mass of B
You can never convert directly from moles of A to grams of B
You must go through the moles!!
One extra step

16. N2 (g) + 3H2 (g)  2NH3 (g)
How many grams of nitrogen must react with 2.3 moles of hydrogen?
Setup:
2.3 mol H2 = _______ g N2
Conversion Factors and/or Mole ratios:
3 mol H2 = 1 mol N2
1 mol N2 = g N2
Work:

17. N2 (g) + 3H2 (g)  2NH3 (g)
How many grams of ammonia are produced if 2.3 moles of hydrogen is reacted with excess nitrogen?
Setup:
2.3 mol H2 = _______ g NH3
Conversion Factors:
3 mol H2 = 2 mol NH3
1 mol NH3= g NH3
Work:

18. Mass A to Mass B You can never go from A to B without moles!! Steps:
1) Convert the initial value to moles
1 mole A = mass grams A
2) Use Mole A to Mole B ratio from equation
3) Convert Moles B to grams of B
1 mole B= mass grams B

19. N2 (g) + 3H2 (g)  2NH3 (g) 15 grams H2 = _______ g NH3
How many grams of ammonia are produced if 15 grams of hydrogen is reacted with excess nitrogen?
Setup:
15 grams H2 = _______ g NH3
Conversion Factors:
1 mol H2 = 2.00 grams H2
3 mol H2 = 2 mol NH3
1 mol NH3= 17.00g NH3
Work:

20. N2 (g) + 3H2 (g)  2NH3 (g)
How many grams of nitrogen reacted if grams of ammonia is produced?
Setup:
100.0 g NH3 = _______ g N2
Conversion Factors:
1 mol NH3= g NH3
2 mol NH3 = 1 mol N2
1 mol N2 = g N2
Work:

21. Mass A to Particles B or Particles A to Mass B
You can never go from A to B without
moles!!
Steps (mass  particles):
1) Convert the initial value to moles
1 mole A = mass grams A
2) Use Mole A to Mole B ratio from equation
3) Convert Moles B to particles of B
1 mole B= 6.02x1023 particles B

22. N2 (g) + 3H2 (g)  2NH3 (g) 20 grams H2 = _______ molecules NH3
How many particles of ammonia are produced if 22 grams of hydrogen is reacted with excess nitrogen?
Setup:
20 grams H2 = _______ molecules NH3
Conversion Factors:
1 mol H2 = 2.00 grams H2
3 mol H2 = 2 mol NH3
1 mol NH3= 6.02x1023 molecules NH3
Work:

23. N2 (g) + 3H2 (g)  2NH3 (g) 7.1x1023 molecules NH3 = _______ g N2
How many grams of nitrogen reacted if 7.1x1023 molecules of ammonia is produced?
Setup:
7.1x1023 molecules NH3 = _______ g N2
Conversion Factors:
6.02x1023 molecules NH3 = 1 mol NH3
2 mol NH3 = 1 mol N2
1 mol N2 = g N2
Work:

24. Mass A to Volume B (gas) 1 mole B= 22.4L B @ STP
To find volume, the reaction must occur at STP.
To go between A and B, you must first find moles!
Steps:
1) Convert the initial value to moles
1 mole A = mass grams A
2) Use Mole A to Mole B ratio from equation
3) Convert Moles B to Liters of B
1 mole B= 22.4L STP

25. N2 (g) + 3H2 (g)  2NH3 (g)
How many grams of nitrogen reacted if 33 liters of ammonia is produced?
Setup:
33 L NH3 = _______ L N2
Conversion Factors:
1 mol NH3= g NH3
2 mol NH3 = 1 mol N2
1 mol N2 = 22.4L STP
Work:

26. N2 (g) + 3H2 (g)  2NH3 (g)
How many liters of ammonia are produced if 55 grams of nitrogen is reacted with excess nitrogen?
Setup:
55 grams N2 = _______ liters NH3
Conversion Factors:
1 mol N2 = grams N2
3 mol H2 = 2 mol NH3
1 mol NH3= 22.4L NH3
Work:

27. Mixed problems Part A
2 H2(g) + O2 (g)  2H20 (g)
How many grams of water are produced if 15 grams of hydrogen are reacted?
Setup:
Conversion factors:
Work:

28. 2 H2(g) + O2 (g)  2H20 (g) Part B
How many liters of oxygen is reacted if 3.4x1023 molecules of water is produced?
Setup:
Conversion factors:
Work:

29. Theoretical vs. Actual Yield
Theoretical yield
Maximum amount of product that can be
made from given amounts of reactants
What you do on paper
Actual yield
The amount of product that ACTUALLY forms when the reaction is carried out in the lab

30. Percent Yield
Equation:
actual yield
theoretical yield
X 100

31. Practice What is the percent yield if the theoretical
yield is 120 grams, but in the experiment only 87 grams is produced?
What is the actual yield if the percent yield is 87% and the theoretical yield is 45 grams?

32. Who’s the BOSS? Limiting Reactant vs. Excess
What is a limiting reactant?
The reactant that LIMITS the reaction
The reactant that is completely used up
Why the boss? B/c the limiting determines how much product is made
What is an excess reactant?
The reactant that is not completely used up

33. Solve for the limiting reactant
2 H2(g) + O2 (g)  2H2O (g)
What is the limiting reactant if 3.2 moles of hydrogen react with 1.55
moles of oxygen react?