1. Starter
A sandwich consists of two slices of bread, 3 slices of meat, and one slice of cheese.
For each of the following amounts, determine the number of sandwiches that can be made and what is left over:
6 bread, 10 meat, 4 cheese slices
10 bread, 6 meat, 8 cheese slices
25 bread, 40 meat, 12 cheese slices

2. Starter 6 bread, 10 meat, 4 cheese slices
3 sandwiches
0 bread, 1 meat, 1 cheese
10 bread, 6 meat, 8 cheese slices
2 sandwiches
6 bread, 0 meat, 6 cheese
25 bread, 40 meat, 12 cheese slices
12 sandwiches
1 bread, 4 meat, 0 cheese

3. Ch. 9 Stoichiometry
9.3 Limiting Reactant

4. Why is there a limiting reactant?
a reaction rarely has exactly the right amount of each reactant
usually have some left over
limiting reactant
reactant that limits the amount of product created
always completely used up
excess reactant
reactant not completely used up

5. When do you have to find a LR?
whenever two amounts of reactants are given in a problem
when only one amount of reactant is given in a problem, then the other is assumed to be in excess

6. Finding Limiting Reactant using reactants
Figure out how much you need of B if you use up all of A
Convert grams A to grams B using stoichiometry
You may start with either reactant
Determine whether you will have enough
If you don’t have enough of B, then B is LR
If you don’t have enough of A, then A is LR

7. Finding Limiting Reactant using products
Convert each of the reactant amounts into an amount of product (doesn’t matter which product)
Compare product amounts and find lowest amount.
Whichever reactant led to lowest product amount is LR

8. Example 1
The reaction begins with 2.51 g of HF and 4.56 g of SiO2. What is the limiting reactant and the excess reactant? How much excess reactant will be left over?
Write the balanced chemical equation
SiO2(s) + 4HF(g)  SiF4(g) + 2H2O(l)

9. Example 1
Find the number of moles available of each reactant:

10. Example 1
If we use up all of the HF, how much SiO2 will we need to go with it?
Do we have enough SiO2?
mol available > mol needed
YES- there will be some left over
Limiting Reactant : HF

11. How much of the product can be formed?
Start conversion with amount of limiting reactant.
Convert to amount of product using stoichiometry

12. Example 1 How many grams of water could be formed?
Convert grams of HF to moles.
Convert moles of HF to moles of water.
Convert moles to grams using molar mass.

13. Example 2 A reaction was done with 36.8 g C6H6 and 41.0 g of O2.
Write the balanced chemical equation
2C6H6 + 15O2  12CO2 + 6H2O
What is the limiting reactant and how much of each product can be produced?

14. Example 2 Reactant Method = 113 g O2 Product Method = 25.5 g H2O
36.8 g C6H6 1 mol C6H6 15 mol O2 32 g O2
78.12 g C6H6 2 mol C6H6 1 mol O2
= 113 g O2
Because 113 g O2 is greater than what we have available, O2 is LR
Product Method
36.8 g C6H6 1 mol C6H6 6 mol H2O g H2O
78.12 g C6H6 2 mol C6H6 1 mol H2O
= 25.5 g H2O
41.0 g O2 1 mol O2 6 mol H2O g H2O
32 g O mol O2 1 mol H2O
= 9.24 g H2O
Because 9.24 is less than 25.5, O2 is the LR

15. Example 2
Once you find the LR, you can then calculate the amount of each product formed…
Amount of H2O
41.0 g O2 1 mol O2 6 mol H2O g H2O
32 g O mol O2 1 mol H2O
= 9.24 g H2O
Amount of CO2
41.0 g O2 1 mol O mol CO g CO2
32 g O mol O2 1 mol CO2
= g CO2

16. Example 3
If the reaction below begins with grams of Fe and 37.5 grams of oxygen, what is the limiting reactant?
4Fe(s) + 3O2(g)  2Fe2O3

17. Example 3
How many grams of oxygen will be left over after the reaction?
How many grams of iron (III) oxide can be formed?