Precipitation Rxns
Solubility, Stoichiometry, and
Net Ionic Equations
Precipitation Rxns: One Kind of Double
Replacement (Metathesis) Rxn
Pb(NO3)2(aq) + 2KI(aq)
PbI2(s) + 2KNO3(aq)
yellow solid
What Happens on the ‘Nanoscale’?
Pb(NO3)2(aq) + 2KI(aq)
PbI2(s) + 2KNO3(aq)
yellow solid

Draw a picture of what is present in
Pb(NO3)2(aq) and KI(aq) before mixing.

Draw a picture of what is present after
mixing.
What Cation-Anion
Combinations Precipitate?

Identify the cation and
anion present in each
solution before mixing.

After mixing, look for
evidence that certain
cations/anions tend to
ppt
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(Investigate This #2.32)
Results of 2.32 Mixing Tests
KNO3
KBr
K2SO4
K2CO3
K2C2O4
K3PO4

Which
combinations
result in pptn?

Which do not?

Categorize
tendency of
each cation and
each anion to
ppt
NaCl
CaCl2
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LiCl
BaCl2
Solubility Rules

From Your Text (p.100-101)

Appendix G of the Lab Manual (more
extensive list)
CoCl2.6H2O and Na3PO4 Solutions
(Investigate This 2.38)
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CoCl2.6H2O(aq)
contains pink
Co2+(aq) and
colorless Cl-(aq)
Na3PO4(aq)
contains colorless
Na+(aq) and
colorless PO43-(aq)
If the Solutions in 2.38 Are Mixed …
mix
Quic kTime™ and a
QuickTime™ and a
TIFF (LZW) decompress or
TIFF (L ZW) d eco mpres sor
are nee ded to s ee this picture. are needed to see this pic ture.
centrifuge
QuickTi me™ a nd a
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are need ed to se e th is p icture.
blue solid forms
What products form?
Na3PO4(aq) + CoCl2(aq) ?
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When Rxn Is Complete

Supernatant contains
only Na+(aq) and Cl-(aq)

Blue ppt of Co3(PO4)2 is
present
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2Na3PO4(aq) + 3CoCl2(aq) Co3(PO4)2 (s) + 6NaCl(aq)
Three Ways to Write Eqn for 2.38

Whole formula equation
2Na3PO4(aq) + 3CoCl2(aq) Co3(PO4)2 (s) + 6NaCl(aq)

Ionic equation
6Na+(aq) + 2PO43-(aq) + 3Co2+(aq) + 6Cl-(aq) 
Co3(PO4)2 (s) + 6Na+(aq) + 6Cl-(aq)

Net ionic equation (excludes spectator ions)
3Co2+(aq) + 2PO43-(aq)  Co3(PO4)2(s)
The Experiment in 2.38
add Na3PO4 (aq)
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CoCl2.6H2O(aq)
3Co2+(aq) + 2PO43-(aq)  Co3(PO4)2(s)
The Experiment in 2.38
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mix
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blue solid forms
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centrifuge
Stoichiometry Calculations (2.38)
Do This For ONE Assigned Tube:

See table on the worksheet for
information about M and V for the
reactant solutions

Determine the number of moles of
each reactant
 moles = M*V
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3Co2+(aq) + 2PO43-(aq)  Co3(PO4)2(s)
More Stoichiometry Calculations
(2.38)
0.00090
mole
0.00045 0.00090 0.00045
mole
mole
mole PO43-
Do This For Assigned Tube:

Identify which reactant will
be consumed entirely
(limiting reactant) and which
will be in excess

Calculate the amount of
each reactant left after rxn

Calculate the yield in moles
of Co3(PO4)2
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0.00090 mole
0.00045 mole Co2+
Reaction Stoichiometry Results
Tube
Co2+ excess or
completely
consumed?
PO43- excess or
completely
consumed?
Moles Co2+
left at end
1
consumed
excess
0
2
excess
3
consumed
excess
0
0.00060 0.00015
4
consumed
excess
0
0.00015 0.00015
consumed 0.00022
Moles PO43left at end
Moles of
Co3(PO4)2
0.00030 0.00030
0
0.00023
Final Questions About 2.38


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
Why is supernatant
in tube #2 pink?
Why are
supernatants in other
tubes colorless?
Why is the amount of
blue ppt not the
same in all the
tubes?
3Co2+(aq) + 2PO43-(aq)  Co3(PO4)2(s)