Fractional Precipitation
Can one type of cation be removed from an aqueous mixture of multiple cations by precipitation?
Why?
In both industry and research there are often times when one particular component of a mixture needs to
be separated from a solution. Maybe it is a rare metal that is dissolved in a mixture of minerals. Maybe it is
a particular protein from lysed plant cells. If the desired component is volatile, distillation could be used.
But if the goal is to separate ions in solution, fractional precipitation is preferred.
Model 1 – A Precipitation Experiment
Solution B
1.00 M Sodium Carbonate
Solution A
1.00 × 10 −6 M Zinc Nitrate
1.00 × 10 −6 M Copper(II) Nitrate
1.00 L
Copper(II) Ion
Selective Electrode
Zinc Ion
Selective Electrode
1. Consider the experiment setup in Model 1.
a. What cations and anions are present in solution A?
b. What is the starting molar concentration of solution A for both zinc and copper(II) ions?
c. Identify the cations and anions present in solution B.
d. What is the concentration of carbonate ions in solution B?
Fractional Precipitation
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2. When solution A and solution B mix, there is potential for two precipitates to form. Write
double replacement reactions to show the formation of the two precipitates.
3. In the experiment setup in Model 1, how will the concentration of zinc and copper(II) ions in
solution A be recorded during the experiment?
4. If solution B is added dropwise to solution A, eventually a precipitate will begin to form. What
do you expect to happen to the concentration of either zinc or copper(II) ions in the solution?
Model 2 – Concentrations of Zn2+ and Cu2+ Ions in Solution A
1.0
0.9
Concentration of Solution A
( × 10−6 M)
0.8
Conc. Zn2+ Ions
in Solution
0.7
Conc. Cu2+ Ions
in Solution
0.6
0.5
0.4
0.3
0.2
0.1
0
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
Volume of Solution B added to Solution A
(mL)
5. Consider the graph in Model 2 that was produced by the experiment in Model 1 as solution B was
added dropwise to solution A. Which line in the graph represents the concentration of zinc ions?
2POGIL™ Activities for AP* Chemistry
6. Initially there is no change in the concentration of either metal cation in solution A. Explain why
this is the case.
7. According to the graph in Model 2, which precipitate is formed first during the experiment in
Model 1—zinc carbonate or copper(II) carbonate? Justify your reasoning.
8. Which solute has the higher solubility—zinc carbonate or copper(II) carbonate? Justify your
reasoning.
9. Suppose a researcher wanted to “pull out” as much zinc ion as possible from solution A without
any copper(II) impurity.
a. How many milliliters of solution B should the researcher add to solution A?
b. What is the maximum percent of the original zinc ion that could be removed from solution A
without any copper(II) impurity?
10. Suppose a researcher wanted to “pull out” as much copper(II) ion as possible from solution A
without any zinc impurity. Could this be achieved by adding solution B? Explain your reasoning.
11. The solubility product constants for the two precipitates that formed when solution A and
solution B are mixed are 5.4 × 10 −11 and 2.5 × 10−10 at 25 °C. Assign these Ksp values to the two
precipitates based on their relative solubilities.
12. Write the Q sp and Ksp expressions for both zinc carbonate and copper(II) carbonate in solution.
Be sure to indicate when initial concentrations are being used in the equation and when equilibrium concentrations are being used in the equation.
Fractional Precipitation
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Model 3 – Comparing Q sp and K sp
Volume of
Solution B
Added (mL)
[CO32−]
Initial
(M)
[Zn2+]
Initial
(M)
ZnCO3
ppt?
(Yes/No)
ZnCO3
Reaction
Quotient
[Cu2+]
Initial
(M)
0.05
1.00 × 10 −6
1.00 × 10 −6
0.10
1.00 × 10 −6
1.00 × 10 −6
0.15
1.00 × 10 −6
1.00 × 10 −6
0.20
1.00 × 10 −6
1.00 × 10 −6
0.25
1.00 × 10 −6
1.00 × 10 −6
0.30
1.00 × 10 −6
1.00 × 10 −6
0.35
1.00 × 10 −6
1.00 × 10 −6
0.40
1.00 × 10 −6
1.00 × 10 −6
CuCO3
ppt?
(Yes/No)
CuCO3
Reaction
Quotient
13. Calculate the initial concentration (before precipitation) of carbonate ions after the addition of
each 0.05 mL of solution B to the 1.00 L beaker of solution A. Divide the work among group
members and write the answers in the table in Model 3. Assume the volume change as solution B
is added is negligible.
14. Notice the initial concentrations of Zn2+ and Cu2+ in the table in Model 3.
a. Explain how these were obtained from the data in Model 2.
b. As solution B is added and precipitates form, do these initial concentrations change?
15. Use the data in Model 2 to indicate the presence of precipitate (either ZnCO3 or CuCO3) after
each 0.05 mL addition of solution B in Model 3.
4POGIL™ Activities for AP* Chemistry
16. Use the initial concentrations of carbonate ions and zinc ions to calculate the reaction quotient,
Q sp, for the zinc carbonate scenarios in Model 3. Divide the work among group members and
write the answers in the table in Model 3.
17. Use the initial concentrations of carbonate ion and copper(II) ions to calculate the reaction
quotient, Q sp, for the copper(II) carbonate scenarios in Model 3. Divide the work among group
members and write the answers in the table in Model 3.
18. Refer to Question 11 for the K sp value for zinc carbonate and copper(II) carbonate. When a precipitate forms in solution, how do Q sp and K sp compare? Use data from Model 3 to support your
answer.
19. Consider the point at which zinc carbonate just begins to precipitate in Model 2.
a. Use the K sp for zinc carbonate to calculate the concentration of carbonate ions required in the
1.00-L beaker of solution A for a precipitate to form.
b. Calculate the volume of solution B that would need to be added to solution A to achieve the
carbonate concentration calculated in part a.
c. Is your answer in part b consistent with the graph in Model 2?
Fractional Precipitation
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20. Consider the point in the experiment when 0.20 mL of solution B has been added to solution A.
a. Calculate the concentration of carbonate ions in the 1.00-L beaker of solution A at this point.
Consider the volume of solution B negligible.
b. Use the K sp of zinc carbonate to calculate the concentration of Zn2+ ions remaining in solution
at this point.
c. Is your answer in part b consistent with the graph in Model 2?
21. Use the K sp for copper(II) carbonate to calculate the volume of solution B that must be added to
solution A to cause a precipitate. Check your answer against the graph in Model 2. Note: Look
back at Question 19 for steps on how to solve this question.
22. Use the K sp of zinc carbonate to calculate the concentration of Zn2+ ions remaining in solution
when copper(II) carbonate begins precipitating. Check your answer against the graph in Model 2.
Hint: Look back at Question 20 for steps on how to solve this question.
6POGIL™ Activities for AP* Chemistry
23. A solution of 2.00 M sodium sulfide is added drop-wise to a solution that is 2.50 × 10 −7 M in
both cobalt(II) ion and nickel(II) ion. The K sp of cobalt(II) sulfide and nickel(II) sulfide are
5.9 × 10 −21 and 3.0 × 10 −21, respectively, at 25 °C.
a. Which precipitate will form first when the sodium sulfide is added? Explain your reasoning.
b. What concentration of sodium sulfide will be required for the first precipitate to form?
c. What concentration of sodium sulfide will be required for the second precipitate to form?
d. When the second precipitate begins to form, what is the concentration of metal ions that
remains unprecipitated from the first precipitate?
Fractional Precipitation
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Extension Questions
24. A 1.00 M solution of lead(II) nitrate is added drop-wise to 1.00 L of 0.0025 M sodium iodide
and 0.0025 M sodium fluoride. The K sp values of lead(II) iodide and lead(II) fluoride are
8.7 × 10 −9 and 3.7 × 10 −8, respectively, at 25 °C.
a. Write the solubility product equations for the precipitates that are expected to form.
b. State the initial concentrations of I1− and F1− in the solution.
c. Which precipitate will form first as lead(II) nitrate is added?
d. What volume of lead(II) nitrate will be required for the first precipitate to form? (Assume the
total change in solution volume is negligible.)
e. What volume of lead(II) nitrate will be required for the second precipitate to form?
f. When the second precipitate begins to form, what is the concentration of halide ion that
remains unprecipitated from the first precipitate?
25. A 0.85 M solution of silver nitrate is slowly added to a solution containing 0.018 M chloride
ions and 0.018 M chromate ions. What is the concentration of chloride ions just as silver chromate begins to precipitate? The K sp values for silver chloride and silver chromate are 1.77 × 10 −10
and 1.12 × 10−12, respectively, at 25 °C.
8POGIL™ Activities for AP* Chemistry