1. Ppt 12 Plan (PS5, 12-18 material)
Limiting Reactant Situations
% Yield
Molarity (done in lab—Exp. 24; worksheet in next Ppt)
Ppt 12

2. Important Realization (about Stoichiometry Problems thus far)
In all stoichiometry problems thus far, you have been TOLD either how much of a
reactant reacts (g or mol), or
product forms (g or mol) A
You were ASKED:
How much of somebody else was formed or used? B
You then used a mole ratio (from the balanced equation) to get
─ moles of B that reacted or formed from moles of A that reacted or formed

3. Example (from earlier “Quiz” worksheet)
If 32.6 g of CCl4 reacts according to the equation below, how many grams of Cl2 will form?
P4O CCl4  CO PCl Cl2
Ppt 12

4. How is this problem different?
If 32.6 g of CCl4 and 11.6 g of P4O10 are combined, and as much reaction occurs as is possible according to the equation below, how many grams of Cl2 will form?
P4O CCl4  CO PCl Cl2
Ppt 12

5. “Limiting Reactant” Situations
You are given amounts of TWO (or more) reactants that are present in a container.
Can all of both reactants react?
Only if they happen to be present (initially) in the same ratio in which they react! VERY RARE!
Usually there will be “leftovers” of one.
So you cannot assume that what is present all reacts
 You need to figure out who runs out “first” and who has some leftover!
“limiting reactant”
it limits the amount of reaction that can occur!
Ppt 12

6. How do you figure out which reactant is limiting?
There are actually many ways, not just one!
Try to understand why these methods “work”
Pick the one best for you or the problem
Ratio method
Assumption method
“Which one make the least amount of product?” method
Ppt 12

7. How do you figure out which reactant is limiting?
1) Ratio method. Directly compare mole ratios: “ratio PRESENT” to “ratio in which they REACT”
Ratio PRESENT > ratio of REACTION
 “too much of substance in NUMERATOR”
 numerator is in excess; denominator is LR
Ratio PRESENT < ratio of REACTION
 “too little of substance in NUMERATOR”
 numerator is LR ; denominator is in excess
Ppt 12

8. How do you figure out which reactant is limiting?
2) Assumption method. Assume one of the reactants (A) ALL REACTS. Calculate how much of the other reactant (B) would be NEEDED (using mole ratio from equation).
moles B PRESENT > moles B NEEDED
 “too much of B” (to react with all the A present)
 B is in excess; A is LR
moles B PRESENT < moles B NEEDED
 “too little of B” (to react with all the A present)
 B is LR ; A is in excess
**Could go “grams A” to “grams of B needed” also
Ppt 12

9. How do you figure out which reactant is limiting?
3) “Which amount of reactants makes the least amount of product?” method a) Assume one of the reactants (A) ALL REACTS. Calculate how much product would be made. b) Do the same thing assuming all of the other reactant (B) ALL REACTS.
 Whichever results in fewer products being made is the L.R.
Ppt 12

10. Example (from "Chemistry, the Molecular Science", Moore, Stanitski, and Jurs (2002, Harcourt) )
CO (g) H2 (g)  CH3OH (l)
(a) Starting with 12.0 g H2 and 74.5 g CO, which reactant is limiting?
(b) What mass of the excess reactant is left over?
(c) What mass of methanol can be obtained (in theory)?
Ppt 12

11. PS Sign-Posting
The concepts and skills related to problems 7-10 on Written PS5 (and some Mastering PS5 problems) were covered in the prior section of this PowerPoint. Give those problems a try now!
Ppt 12

12. If masses of both reactants are given (“Limiting Reactant” situation)
“Theoretical yield”
(Maximum possible assuming all of the LR reacted in the way you assumed)
Compare the mole ratio of reactants PRESENT to the mole ratio of REACTION (from equation)
[or use “assumption” or “which makes the least amount of products” method]
Because it all reacted!
Formed, in theory OR
moles of other reactant that reacted!!
Ppt 12

13. Actual vs. Theoretical Yield: Theory calculates the max, but you rarely get that in the real world!
"yield" means "amount of a product“
- it could be in grams or moles
“theoretical yield" means "maximum amount of product”
- Assumes that as much reaction as possible occurs according to a given balanced chemical equation).
 no “side” reactions occur
 no losses during isolation (stuck to filter paper, etc.)
- Obtained by doing a stoichiometry calculation using the balanced equation
“actual yield" means “amount of product you actually isolated in the lab after doing a reaction”
Ppt 12

14. % Yield: How well did you do?!
“% Yield” means “percent of the maximum amount that you actually got”
Warning: Read problems carefully! You might not be asked for % yield!
- If you have any 2 of the 3 “variables” you can calculate the third! 14

15. Example (from "Chemistry, the Molecular Science", Moore, Stanitski, and Jurs (2002, Harcourt) )
CO (g) H2 (g)  CH3OH (l)
What is the percent yield of a reaction in which 5.0 x 103 g of H2 reacts with excess CO to form 3.5 x 103 g CH3OH?
Given: Actual yield (“to form”); Asked for: % Yield
 Needed: Theoretical yield
Ppt 12

16. Stoichiometry Quiz-Revisited (Chemistry 121 Quiz Used as PRACTICE WORKSHEET on "Early" Stoichiometry)
1. (8 pts) Given the following chemical equation:
P4O CCl4  CO PCl Cl2
How many moles of CO2 will be formed in theory if 3 moles of P4O10 react?
How many moles of P4O10 would be used up if 1.9 moles of Cl2 were produced?
How many moles of PCl3 form in theory if 2.4 moles of CCl4 react?
How many moles of CO2 will be formed in theory if moles of P4O10 react?
How many grams of Cl2 would be formed in theory if 3.2 moles of P4O10 were reacted?
If 32.6 g of CCl4 reacts, how many grams of Cl2 form in theory?
***It is implied in all these problems that you are determining a theoretical yield!***
Ppt 12 16

17. PS Sign-Posting
The concepts and skills related to some problems in PS5 in Mastering were covered in the prior section of this PowerPoint (no % yield problems ended up on the written part of PS5—make sure not to skip Mastering problems!). Give those problems a try now!
Ppt 12