Limiting Reagents

1
Limiting Reagents

• Caution this stuff is difficult to follow at
  first.
• Be patient.

2
Balloon Flask Demonstration
g of NaHCO3 mL of 3M HCl
1 10 25
2 10 50
3 10 100
How can we prove that our conclusions about
limiting reagents is correct?
3
Limiting reagent defined
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

4 mol NO, works out exactly
4 mol NO, with leftover O2
8 mol NO, with leftover O2

• Here, NH3 limits the production of NO if there
  was more NH3, more NO would be produced
• Thus, NH3 is called the limiting reagent
• 4 mol NH3, 2.5 mol O2

2 mol NO, leftover NH3

• In limiting reagent questions we use the limiting
  reagent as the given quantity and ignore the
  reagent that is in excess

4
Limiting reagents in stoichiometry
4NH3 5O2 ? 6H2O 4NO

• E.g. How many grams of NO are produced if 4
  moles NH3 are burned in 20 mol O2?
• Since NH3 is the limiting reagent we will use
  this as our given quantity in the calculation

g NO
4 mol NH3
120 g NO

• Sometimes the question is more complicated. For
  example, if grams of the two reactants are given
  instead of moles we must first determine moles,
  then decide which is limiting

5
Solving Limiting reagents 1 g to mol
4NH3 5O2 ? 6H2O 4NO

• Q - How many g NO are produced if 20 g NH3 is
  burned in 30 g O2?
• A - First we need to calculate the number of
  moles of each reactant

mol NH3
20 g NH3
mol O2
30 g O2
A Once the number of moles of each is
calculated we can determine the limiting reagent
via a chart
6
2 Comparison chart
NH3 O2
What we have

What we need

1.176
0.937
1.176/0.937 1.25 mol
0.937/0.937 1 mol
4
5
4/5 0.8 mol
5/5 1 mol
Choose the smallest value to divide each by
You should have 1 mol in the same column
twice in order to make a comparison
A - There is more NH3 (what we have) than needed
(what we need). Thus NH3 is in excess, and O2 is
the limiting reagent.
7
3 Stoichiometry (given limiting)

• So far we have followed two steps
• 1) Expressed all chemical quantities as moles
• 2) Determined the limiting reagent via a chart
• Finally we need to
• 3) Perform the stoichiometry using the limiting
  reagent as the given quantity
• Q - How many g NO are produced if 20 g NH3 is
  burned in 30 g O2?
• 4NH3 5O2 ? 6H2O 4NO

g NO
30 g O2
8
Limiting Reagents shortcut

• Limiting reagent problems can be solved another
  way (without using a chart)
• Do two separate calculations using both given
  quantities. The smaller answer is correct.
• Q - How many g NO are produced if 20 g NH3 is
  burned in 30 g O2? 4NH3 5O2? 6H2O 4NO

g NO
20 g NH3
30 g O2
9
Practice questions

• 2Al 6HCl ? 2AlCl3 3H2
• If 25 g of aluminum was added to 90 g of HCl,
  what mass of H2 will be produced (try this two
  ways with a chart using the shortcut)?
• N2 3H2 ? 2NH3 If you have 20 g of N2 and 5.0 g
  of H2, which is the limiting reagent?
• What mass of aluminum oxide is formed when 10.0 g
  of Al is burned in 20.0 g of O2?
• 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?
• How can you tell if a question is a limiting
  reagent question vs. typical stoichiometry?

10
1
mol Al
25 g Al
0.926 mol
mol HCl
90 g HCl
2.466 mol
Al HCl
What we
have
What we
need
0.926
2.466
HCl is limiting.
2.466/0.926 2.7 mol
0.926/0.926 1 mol
2
6
2/2 1 mol
6/2 3 mol
g H2
90 g HCl
2.47 g H2
11
Question 1 shortcut

• 2Al 6HCl ? 2AlCl3 3H2
• If 25 g aluminum was added to 90 g HCl, what mass
  of H2 will be produced?

g H2
25 g Al
g H2
90 g HCl
12
Question 2 shortcut

• N2 3H2 ? 2NH3
• If you have 20 g of N2 and 5.0 g of H2, which is
  the limiting reagent?

g NH3
20 g N2
g NH3
5.0 g H2
N2 is the limiting reagent
13
Question 3 shortcut

• 4Al 3O2 ? 2 Al2O3
• What mass of aluminum oxide is formed when 10.0 g
  of Al is burned in 20.0 g of O2?

g Al2O3
10.0 g Al
g Al2O3
20.0 g O2
14
Question 4 shortcut

• C3H8 5O2 ? 3CO2 4H2O
• 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?

g CO2
15.0 g C3H8
g CO2
60.0 g O2
5. Limiting reagent questions give values for
two or more reagents (not just one)
15
Question 2
mol N2
20 g N2
mol H2
5.0 g H2
N2 H2
What we have

What we need
0.714 mol
2.5 mol
0.714/0.714 1 mol
2.5/0.714 3.5 mol
1 mol
3 mol
We have more H2 than what we need, thus H2 is in
excess and N2 is the limiting factor.
16
3

• 4Al 3O2 ? 2 Al2O3

mol Al
10 g Al
mol O2
20 g O2
There is more than enough O2 Al is limiting
Al O2




0.37 mol
0.625 mol
0.37/.37 1 mol
0.625/0.37 1.68 mol
4 mol
3 mol
4/4 1 mol
3/4 0.75 mol
g Al2O3
0.37 mol Al
17
4

• C3H8 5O2 ? 3CO2 4H2O

mol C3H8
15 g C3H8
mol O2
60 g O2
We have more than enough O2, C3H8 is limiting
C3H8 O2



0.34 mol
1.875 mol
What we have
0.34/.34 1 mol
1.875/0.34 5.5 mol
Need
1 mol
5 mol
g CO2
0.34 mol C3H8
18
Limiting Reagents shortcut

• MgCl2 2AgNO3 ? Mg(NO3)2 2AgCl
• If 25 g magnesium chloride was added to 68 g
  silver nitrate, what mass of AgCl will be
  produced?

g AgCl
25 g MgCl2
g AgCl
68 g AgNO3
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