Solubility Equilibria And The Solubility Product Constant (Chemistry Worksheet) Page 2
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1
2
3M
= 2(108.98 g/mol) + 3(32.07 g/mol) = 514.17 g/mol
Bi2S3
-14
-16
Molar Solubility = 4.68 x 10
g x (1000 ml) x (1 mol) = 9.10 x 10
mol/L
100 mL
1 L
541.17 g
3
3+
2
2-
]
K
= [Bi
]
[S
sp
(aq)
(aq)
(Q) is the product of the concentrations of ions in specific solutions raised
to powers (equal to their coeffiecients). They are used to predict the formation of a precipitate
(the forward direction of a solubility equilibrium). Q is calculated in the same way that K
is.
sp
2+
-
2+
-
Use the example of CaF
⇌ Ca
+ F
Q = [Ca
][F
]
2(s)
(aq)
(aq)
(aq)
(aq)
The Three Possibilities:
1. Q = K
; In this case, the system has established _________________. No changes occur
sp
2. Q < K; System needs to shift to the _________ in order to establish equilibrium. The solution is
unsaturated and no precipitation occurs.
3. Q > K; System needs to shift to the ________ in order to establish equilibrium. The solution is saturated
and precipitation occurs until a new equilibrium is reached.
Example Problem 4: Will a precipitate form if a student mixes 100.0 mL of an aqueous solution
of 0.100 mol/L CaCl
with 100.0 mL of 0.0400 mol/L Na
SO
. If so, identify the precipitate?
2(aq)
2
4(aq)
Step 1: Include Given Information and what we are trying to find
G: V
= 100.0 L, [CaCl
] = 0.100 mol/L, V
= 100.0 mL
[Na
SO
] = 0.0400 mol/L
CaCl2
2
Na2SO4
,
2
4
R: Identity of the Precipitate = ?
Step 2: Use the solubility chart to determine if a precipitate will form
The two potential products in this reaction are NaCl and CaSO
. According to your solubility
4
chart, NaCl is highly soluble and CaSO
has a low solubility so CaSO
is likely the precipitate.
4
4
Step 3: Determine the concentration of ions that may form a precipitate based on the original
reactants. There is a 1:1 mole ratio between the ionic compound and their ions needed to form
the precipitate
2+
-
+
2-
CaCl
⇌ Ca
+ 2 Cl
Na
SO
⇌ 2Na
+ SO
2(s)
(aq)
(aq)
2
4(s)
(aq)
4
(aq)
2+
2-
[Ca
]
= 0.100 mol/L
[SO
]
= 0.0400 mol/L
(aq)
initial
4
(aq)
initial
Step 4: The initial volumes are different from the final volume of the solution. Use the dilution
formula to determine the concentration of the ions from the combined volume.
2+
V
V
+ V
c
v
= c
v
[Ca
]
=
f =
CaCl2
Na2SO4
1
1
2
2
final
V
100.0 L + 100.0 L
c
= c
v
/v
f =
2
1
1
2
2-
= 200.0 L
[SO
]
=
4
final
Step 5: Determine the Ion Product Equation for the Precipitate, Calculate Q and Compare K
& Q
sp
2+
2-
2+
2-
CaSO
⇌ Ca
+ SO
Q = [Ca
][SO
]
4(s)
(aq)
4
(aq)
(aq)
4
(aq)
Q =
-5
K
= 7.1 x 10
sp
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