1. Stoichiometric Calculations
Stoichiometry
Stoichiometric Calculations

2. A. Proportional Relationships
2 1/4 c. flour
1 tsp. baking soda
1 tsp. salt
1 c. butter
3/4 c. sugar
3/4 c. brown sugar
1 tsp vanilla extract
2 eggs
2 c. chocolate chips
Makes 5 dozen cookies.
I have 5 eggs. How many cookies can I make?
Ratio of eggs to cookies
5 eggs
5 doz.
2 eggs
= 12.5 dozen cookies

3. A. Proportional Relationships
Stoichiometry
mass relationships between substances in a chemical reaction
based on the mole ratio
Mole Ratio
indicated by coefficients in a balanced equation
2 Mg + O2  2 MgO

4. What can we do with Stoichiometry?
For the generic equation:
RA +RB → P1 + P2
Given the…
…one can find the…
Amount of RA (or RB)
Amount of RA (or RB) that is needed to react with it
Amount of RA or RB
Amount of P1 or P2 that will be produced
Amount of P1 or P2 you need to produce
Amount of RA &/or RB you must use

5. Given the equation… 2TiO2 + 4Cl2 + 3C → 2TiCl4 + CO2 + 2CO
How many mol chlorine will react with 4.55 mol carbon?
X mol Cl2 = 4.55 mol C (4mol Cl2/ 3mol C)
6.07 mol Cl2 will react with 4.55 mol carbon

6. Given the same equation…
2TiO2 + 4Cl2 + 3C → 2TiCl4 + CO2 + 2CO
What mass titanium (IV) oxide will react with 4.55 mol carbon?
X g TiO2 =
4.55 mol C(2 mol TiO2/3 mol C)(79.9 g TiO2/1 molTiO2)
242 g TiO2 will react with 4.55 mol carbon

7. And with the same equation…
2TiO2 + 4Cl2 + 3C → 2TiCl4 + CO2 + 2CO
How many molecules titanium (IV) chloride can be made from 115 g titanium (IV) oxide?
X m’c TiCl4 =
115g TiO2 (1mol TiO2/79.9g TiO2)(2 mol TiCl4/2 mol TiO2)(6.02 x 1023 m’c TiCl4/1 mol TiCl4)
8.7 x 1023 m’cules TiCl4 can be made from 115 g TiO2

8. B. Stoichiometry Steps 1. Write a balanced equation.
2. Identify known & unknown.
3. Line up conversion factors.
Mole ratio - moles  moles
Molar mass - moles  grams
Molarity - moles  liters soln
Molar volume - moles  liters gas
Mole ratio - moles  moles
Core step in all stoichiometry problems!!
4. Check answer.

9. Standard Temperature & Pressure
C. Molar Volume at STP
1 mol of a gas=22.4 L
at STP
Standard Temperature & Pressure
0°C and 1 atm

10. C. Molar Volume at STP LITERS OF GAS AT STP Molar Volume MASS IN GRAMS
(22.4 L/mol)
MASS IN
GRAMS
MOLES
NUMBER OF
PARTICLES
Molar Mass
(g/mol)
6.02  1023
particles/mol
Molarity (mol/L)
LITERS OF
SOLUTION

11. Stoichiometry “Pop” Quiz
Iron (III) reacts with water…
Write the complete balanced equation
How many liters of hydrogen STP) will be produced from 8 moles of iron?
How many grams of water are needed to produce 68 grams of Iron (III) oxide?
5.2 x 1025 particles of iron will produce how many moles of hydrogen?
For the complete combustion of butane (C4H10)…
In a room at 0°C and 1 atm pressure, how many m’cules oxygen react with 632 L butane?
If 316 L liters of carbon dioxide gas are produced, how much butane (in moles) was combusted?
Silver nitrate reacts with sodium chloride…
Write the complete balanced equation (including states of matter)
How much precipitate (in g) is formed when 1.5 L of 0.10M silver nitrate is used?
How many molecules of sodium chloride do you need to react with that amount of silver nitrate?

12. The Limiting Reactant
A balanced equation for making a Big Mac® might be:
3 B M + EE  B3M2EE
With…
…and…
…one can make…
30 M
excess B and excess EE
15 B3M2EE
30 B
excess M and excess EE
10 B3M2EE
30 B and excess EE

13. A balanced equation for making a tricycle might be:
3 W P + S + H + F  W3P2SHF
With…
…and…
…one can make…
50 P
excess of all other reactants
25 W3P2SHF
50 S
50 W3P2SHF
50 S and excess of all other reactants

14. Solid aluminum reacts w/chlorine gas to yield solid aluminum chloride.
2 Al(s) Cl2(g)  AlCl3(s)
If 125 g aluminum react w/excess chlorine, how many g aluminum chloride are made?
= g AlCl3

15. If 125 g chlorine react w/excess aluminum, how many g aluminum chloride are made?
= g AlCl3
If 125 g aluminum react w/125 g chlorine, how many g aluminum chloride are made? g AlCl3
We’re out of Cl2.

16. limiting reactant (LR): the reactant that runs out first
Amount of product is based on LR.
Any reactant you don’t run out of is an excess reactant (ER).

17. Example
Limiting Reactant
Excess Reactant(s)
Big Macs
buns
meat
tricycles
pedals
W, S, H, F
Al / Cl2 / AlCl3
Cl2 Al

18. How to Find the Limiting Reactant
For the generic reaction RA RB  P
Assume that the amounts of RA and RB are given.
Should you use RA or RB in your calculations?

19. How to find LR steps… 1. Calc. # of mol of RA and RB you have.
2. Use the mole ratio from the balanced equation to calculate the required amount of the other reactant.
3. If resulting amount is less than what is given, that reactant is the LR.

20. % Yield 6 Na(l)+ 1 Al2O3(s)  2 Al(l) + 3 Na2O(s)
molten solid  molten solid
sodium aluminum aluminum sodium
oxide oxide
Al3+ O2– Na1+ O2–
___Na(l) + ___Al2O3(s)  ___Al(l) +___Na2O(s)
6 Na(l)+ 1 Al2O3(s)  2 Al(l) + 3 Na2O(s)
Find mass of aluminum produced if you start w/575 g sodium and 357 g aluminum oxide.
= 189 g Al

21. % Yield This amount of product is the theoretical yield.
amt. we get if reaction is perfect
found by calculation
Now suppose that we perform this reaction in the lab and get only 172 grams of aluminum. Why?
couldn’t collect all Al
not all Na and Al2O3 reacted
some reactant or product spilled and was lost

22. % yield can never be > 100%
% Yield Calculation:
% yield can never be > 100%

23. Find % Yield for the Previous Problem