Download Precipitate or not

Precipitate or
not
• Ksp values can also be used to
predict if mixing solutions will
produce a precipitate.
Key Idea
• When the product of a concentration of
ions, for two substances in a mixture
as described by the equilibrium
constant expression exceeds the value
of the Ksp, a precipitate will form.
• Ie: If Q > Ksp
a precipitate will form.
Steps
1. Balanced chemical equation.
2. Find the moles of each reactant.
3. Find the total volume ( add the volume of the
reactants).
4. Find the molarity of the reactants
(moles/total volume).
5. Dissociate each reactant, find the
concentration of each ion.
6. Write the Ksp expression of the solid.
7. Find the trial Ksp or Q.
8. Is Q > Ksp, if so a precipitate forms.
•
Will a precipitate of Silver chloride form if
one drop of (0.050 ml) of 5.0 M silver
nitrate solution is added to 1.5 L of 0.20
M sodium chloride,
if the Ksp is 1.8 x 10 -8 ?
1. Balance the equation
AgNO3 (aq) + NaCl (aq)→ AgCl (s) +NaNO3 (aq)
2. moles of each
AgNO3
0.05 ml of 5.0 M
(0.00005 l)(5.0 mol/l)
= 0.00025 mol
NaCl
1.5 l of 0.20 M
(1.5 l)(0.20 mol/l)
= 0.30 mol
3. Total volume is 1.5 l
( add the two volumes together)
4. Molarity of reactants
AgNO3
NaCl
0.00025 mol
1.5 L
0.30 mol
1.5 L
= 1.67 x 10-4 mol/l
= 0.20 mol/l
5. dissociation
→[Ag+]
+
1.67 x 10-4 M
→ [Na+]
0.20 M
[NO3-]
1.67 x 10-4 M
+
[Cl-]
0.20 M
6. Write Ksp expression for
silver chloride.
• AgCl (s) ------> Ag+ (aq) + Cl- (aq)
7. Solve for Q
Ksp = [Ag + (aq) ] [Cl - (aq)]
=( 1.67 x 10-4)(0.20)
= 3.34 x 10-5
8. Compare Q to Ksp
• If Q > ksp
• 3.34 x 10-5 > 1.8 x 10-8
• A ppt will form.
• Determine if a precipitate of Ca(OH)2
will form is 20 ml of 0.30 mol/l CaCl2
solution is mixed with 10 ml of 0.90
mol/l NaOH, if the Ksp for Ca(OH)2 is
5.1 X 10-6.
solution
CaCl2 (aq)
+
20 ml of 0.30M
2 NaOH (aq)
10 ml of 0.90M
→ Ca(OH)2 (s) + 2 NaCl (aq)
CaCl2 (aq)
+
(0.020 l)( 0.30 mol/l)
= 0.06 mol
0.030 L
= 0.02 M
→Ca2+ +
0.02
2 NaOH (aq)
(0.010 l)( 0.90mol/l)
=0.009 mol
0.030 L
= 0.30 M
2Cl0.04
→ 2 Na+
0.30
+ 2 Cl0.30
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Ksp expression for Ca(OH)2 is:
Ca(OH)2 ----> Ca 2+ (aq) + 2 OH- (aq)
Ksp = [Ca 2+ ] [OH-] 2
Q = ( 0.20)(0.30)2
Q = 0.018
Ksp = 5.1 x 10 -6
Q > Ksp therefore a ppt forms