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Limiting Reagent
If you are given one dozen loaves of bread, a gallon of mustard and three pieces of salami, how many salami sandwiches can you make?
Limiting Reagent
The limiting reagent is the reactant you run out of first.
The excess reagent is the one you have left over.
The limiting reagent determines how much product you can make.
Determining the Limiting Reactant
Do two stoichiometry problems. The one that makes the least product
is the limiting reagent.
Example
Copper reacts with sulfur to form copper (I) sulfide. If 10.6 g of copper reacts with 3.83 g sulfur, how much product will be formed?
2 Cu + S Cu2S
10.6 g Cu 63.5 g Cu
1 mol Cu
2 mol Cu
1 mol Cu2S 1 mol Cu2S
159.1 g Cu2S
= 13.3 g Cu2S
Example
2 Cu + S Cu2S
3.83 g S 32.1 g S
1 mol S1 mol S
1 mol Cu2S 1 mol Cu2S
159.1 g Cu2S
= 19.0 g Cu2S
Example
2 Cu + S Cu2S
Example
10.6 grams of copper resulted in 13.3 grams of copper (I) sulfide.
3.83 grams of sulfur resulted in 19.0 grams of copper (I) sulfide.
2 Cu + S Cu2S
Cu is the limiting reactant!
Example
Using 10.6 grams of copper resulted in the smaller amount of copper (I) sulfide formed. Therefore
2 Cu + S Cu2S
Example
Considering the limiting reactant, 13.3 grams of copper (I) sulfide are formed.
Example
Determine the mass of P4O10 formed if 25.0 g of P4 and 50.0 g of O2 are combined. Which reactant is the limiting reactant?
Write the balanced equation P4 + 5 O2 P4O10
Example
P4 + 5 O2 P4O10
25.0 g P4
124.0 P4
1 mol P4
1 mol P4
1 mol P4O10
1 mol P4O10
284.0 g P4O10
= 57.3 g P4O10
Example
P4 + 5 O2 P4O10
50.0 g O2
32.0 O2
1 mol O2
5 mol O2
1 mol P4O10
1 mol P4O10
284.0 g P4O10
= 88.8 g P4O10
Example
Example
P4 is the limiting reactant, and 57.3 grams of P4O10 are formed.
Problem
The reaction between solid sodium and iron (III) oxide is one in a series of reactions that inflates an automobile airbag. If 100. g of Na and 100. g of Fe2O3 are used in the reaction, determine the limiting reactant and the mass of solid iron produced. Fe2O3 limits, 69.9 g
of Fe produced
Problem
How many moles of cesium xenon heptafluoride (CsXeF7) can be produced from the reaction of 12.5 mol of cesium fluoride with 10.0 mol of xenon hexafluoride?
10.0 mol
Problem
The reaction of chlorine gas with solid phosphorus (P4) produces solid PCl5. When 22.0 g of chlorine reacts with 25.5 g of P4, which reactant limits the amount of product?
Cl2 is limiting.
How Much Excess Reagent?
Convert the limiting reagent into the excess reagent.
Subtract that from the amount of excess you started with.
Example
Mg(s) + 2 HCl(g) MgCl2(s) + H2(g) If 10.1 mol of magnesium and
4.87 mol of HCl gas are reacted, how many moles excess reagent remain?
Example
Mg(s) + 2 HCl(g) MgCl2(s) + H2(g) First, determine which reactant is in
excess.
Mg(s) + 2 HCl(g) MgCl2(s) + H2(g)
10.1 mol Mg
1 mol Mg
1 mol MgCl2 = 10.1 mol MgCl2
Example
Mg(s) + 2 HCl(g) MgCl2(s) + H2(g)
4.87 mol HCl
2 mol HCl
1 mol MgCl2 = 2.44 mol MgCl2
Example
Example
Mg is in excess. Now determine how much
excess! Convert the limiting reactant to
excess reactant.
Mg(s) + 2 HCl(g) MgCl2(s) + H2(g)
4.87 mol HCl
2 mol HCl
1 mol Mg = 2.44 mol Mg
Example
Example
Now take the excess reactant and subtract the liming converted into excess.
10.1 mol Mg – 2.44 mol Mg =
7.66 mol of Mg are in excess
Example
If 10.3 g of aluminum are reacted with 51.7 g of CuSO4 how much excess reagent will remain?
Example
Write the balanced equation. If 10.3 g of aluminum are reacted
with 51.7 g of CuSO4 how much excess reagent will remain?
2Al + 3CuSO4 Al2(SO4)3 + 3Cu
Example
2Al + 3CuSO4 Al2(SO4)3 + 3Cu First, determine which reactant is in excess.
2Al + 3CuSO4 Al2(SO4)3 + 3Cu
10.3 g Al
27.0 g Al
1 mol Al
2 mol Al
3 mol Cu
= 0.572 mol Cu
Example
2Al + 3CuSO4 Al2(SO4)3 + 3Cu
51.7 g CuSO4
159.6 g CuSO4
1 mol CuSO4
3 mol CuSO4
3 mol Cu
= 0.324 mol Cu
Example
Example
Al is in excess. Now determine how much
excess! Convert the limiting reactant to
excess reactant.
2Al + 3CuSO4 Al2(SO4)3 + 3Cu
51.7 g CuSO4
159.6 CuSO4
1 mol CuSO4
3 mol CuSO4
2 mol Al
1 mol Al
27.0 g Al
= 5.83 g Al
Example
Example
Now take the excess reactant and subtract the liming converted into excess.
10.3 g Al – 5.83 g Al =
4.47 g of Al are in excess
Problem
Zn + 2MnO2 + H2O Zn(OH)2 + Mn2O3
If 25.0 g of zinc and 39.0 g of manganese dioxide are used, how much excess reagent will remain?
(Answer: 10.3 g Zn)
Problem
Lithium reacts spontaneously with bromine to produce lithium bromide. If 3.40 g of lithium react with 32.0 g of bromine, how much excess reactant remains?
(Answer: 0.64 g Li)
Problem
CH4 + 3Cl2 CH3Cl + 3HCl If 50.0 g of methane reacts with
9.67 moles of chlorine gas, how mass of excess reagent will remain?
(Answer: 20.6 g of Cl2)
Yield
The yield is the amount of product made in a chemical reaction.
There are three types of yield that can be determined.
Yield Actual yield - what you get in the lab
when the chemicals are mixed
Theoretical yield - what the balanced equation tells you you should make
Actual Percent yieldPercent yield = x 100 %
Theoretical
Percent Yield
Percent yield tells us how “efficient” a reaction is.
Percent yield cannot be larger than 100 %.
Example
6.78 g of copper are produced when 3.92 g of Al are reacted with excess copper (II) sulfate.
2Al + 3 CuSO4 Al2(SO4)3 + 3Cu
What is the actual yield of copper?
(Answer: 6.78 g)
Example
6.78 g of copper are produced when 3.92 g of Al are reacted with excess copper (II) sulfate.
2Al + 3 CuSO4 Al2(SO4)3 + 3Cu
What is the theoretical yield?
2Al + 3 CuSO4 Al2(SO4)3 + 3Cu
3.92 g Al
27.0 Al
1 mol Al
2 mol Al
3 mol Cu
1 mol Cu
63.5 g Cu
= 13.8 g Cu
Example
Example
6.78 g of copper is produced when 3.92 g of Al are reacted with excess copper (II) sulfate.
2Al + 3 CuSO4 Al2(SO4)3 + 3Cu
What is the percent yield?
Yield
Actual Percent yieldPercent yield = x 100 %
Theoretical
6.78 g Percent yieldPercent yield = x 100 %
13.8 g
= 49.1%
Problem
58.2 g of zinc are produced when 14.7 g of Li are reacted with excess zinc sulfate.
2Li + ZnSO4 Li2SO4 + Zn
What is the actual yield of zinc?
(Answer: 58.2 g)
Problem
58.2 g of zinc are produced when 14.7 g of Li are reacted with excess zinc sulfate.
2Li + ZnSO4 Li2SO4 + Zn
What is the theoretical yield of zinc?
(Answer: 69.7 g)
Problem
58.2 g of zinc are produced when 14.7 g of Li are reacted with excess zinc sulfate.
2Li + ZnSO4 Li2SO4 + Zn
What is the percent yield of zinc?
(Answer: 83.5%)