6.3 Limiting Reagents
General idea:
• So far we have assumed that all of the reactants taking part in the reaction have been
transformed into products. (No left-overs)
• Not usually the case.
Ex. Campfires:
 Unlimited oxygen in the air available for the reaction
(oxygen = __________________)
 When all wood is burned, the reaction stops, no longer
using oxygen from the air, and making no more water or
CO2.
 The amount of wood limits the amount of products,
therefore, it is the ______________
Ex. Baking a Cake

The recipe calls for 2 eggs to combine with 1 cup of flour
and this make 1 cake.

How many cakes could you bake if you have 4 eggs in
your fridge, and 4 cups of flour in your pantry?

To use up all 4 eggs, you would only need 2 cups of flour.
(2 cups left over).

Flour = ___________________

Eggs = ___________________
In Chemistry:
Given: 4NH3 + 5O2 → 6H2O + 4NO
Q - How many moles of NO are produced if __ mol NH3 are burned in __ mol O2?
4 mol NH3, 5 mol O2
4 mol NH3, 20 mol O2
8 mol NH3, 20 mol O2
__________________________
__________________________
__________________________
In all of these cases, NH3 _______ the production of NO; if there was more ______,
______ NO would be produced
Thus, NH3 is called the “_____________________”
…………………………………………………………………………
4 mol NH3, 2.5 mol O2
__________________________
Here, O2 limits the production of NO; if there was more ___, ______ NO would be
produced
Thus, O2 is called the “_____________________”
STOICHIOMETRY
CHEMISTRY 20
6.3 Limiting Reagents
Procedure for solving limiting reagent problems
1.
2.
3.
4.
5.
If not already, balance the chemical equation.
Convert information given into units of moles.
Divide the mole amount of each reactant by its stoichiometric
coefficient from the balanced chemical equation.
The reactant with the smallest quotient from step #3 will be the
limiting reagent.
Work as typical stoichiometry problem using the limiting reagent.
Example #1
Given the following balanced chemical equation:
2Ca + O2 → 2CaO
How many moles of calcium oxide (CaO) can you form starting with 15 moles of calcium (Ca)
and 12 moles of oxygen (O2)?
1.
If not already, balance the chemical equation.
2.
Convert information given into units of moles.
3.
Divide the mole amount of each reactant by its stoichiometric ratio from the balanced
chemical equation.
4.
The reactant with the smallest quotient from step #3 will be the limiting reagent.
5.
Work as typical stoichiometry problem using the limiting reagent.
STOICHIOMETRY
CHEMISTRY 20
6.3 Limiting Reagents
Example #2
What mass of CO2 can be produced by the reaction of 8.0 grams of CH4 with 48 grams of O2
according to the equation below?
CH4 + 2O2 → CO2 + 2H2O
1.
If not already, balance the chemical equation.
2.
Convert information given into units of moles.
3.
Divide the mole amount of each reactant by its stoichiometric ratio from the balanced
chemical equation.
4.
The reactant with the smallest quotient from step #3 will be the limiting reagent.
5.
Work as typical stoichiometry problem using the limiting reagent.
Limiting Reagents: Shortcut
Do two separate calculations using both given quantities. The __________ answer is correct.
Example #3
How many g NO are produced if 20 g NH3 is burned in 30 g O2?
4 NH3 + 5 O2→ 6H2O+ 4NO
6.3 Limiting Reagents
Limiting Reagents Assignment
1. How can you tell if a question is a limiting reagent question vs. typical stoichiometry?
2. If 25 g of aluminum was added to 90 g of HCl, what mass of H2 will be produced (try this two ways – with
the steps & using the shortcut)? [2.47g H2]
3. N2 + 3H2 → 2NH3: If you have 20 g of N2 and 5.0 g of H2, which is the limiting reagent? [N2]
4. What mass of aluminum oxide is formed when 10.0 g of Al is burned in 20.0 g of O 2? [18.9g Al2O3]
5. When C3H8 burns in oxygen, CO2 and H2O are produced. If 15.0 g of C3H8 reacts with 60.0 g of O2, how
much CO2 is produced? [45.0g CO2]
STOICHIOMETRY
CHEMISTRY 20
6.3 Limiting Reagents
6. How many grams of NH3 can be produced from the reaction of 28g of N2 and 25g of H2? [34g NH3]
7. How much of the excess reagent in Problem 6 is left over? [19g H2 in excess]
8. What volume of hydrogen at STP is produced from the reaction of 50.0g of Mg and the equivalent of 75g of
HCl? [23.0L H2]
9. How much of the excess reagent in Problem 8 is left over? [25g Mg in excess]
10. Silver nitrate and sodium phosphate are reacted in equal amounts of 200g each. How many grams of
silver phosphate are produced? [164.3g Ag3PO4]
11. How much of the excess reagent in problem 5 is left? [135.6g Na3PO4]
STOICHIOMETRY
CHEMISTRY 20
6.3 Limiting Reagents
12. Iron reacts with sulfur to form iron(II) sulfide. You have 32.0g of sulfur and 100g of iron. Calculate the
number of grams of iron(II) sulfide that will form. [87.9g FeS]
13. Zinc reacts with sulfuric acid in a single replacement reaction. You have 40g of zinc and 57g of sulfuric
acid. What volume of hydrogen gas will form? [13.0L H2]
14. Silver nitrate and sodium bromide will react in a double replacement reaction. You have 24g of silver
nitrate and 39g of sodium bromide. How many grams of sodium nitrate will form? [12g NaNO3]
15. When hydrochloric acid reacts with iron(II) sulfide, a double replacement reaction is to be expected. You
have 67g of hydrochloric acid and 58 of iron(II) sulfide. How many grams of hydrogen sulfide will form?
[22.5g H2S]
16. When aluminum is heated in oxygen, aluminum oxide is formed. You have 384g of each reactant. How
many moles of aluminum oxide will form? [7.12mol Al2O3]
STOICHIOMETRY
CHEMISTRY 20