Problems A
385
(a) Cu
2+
(d) Sn
2+
(b) Cr
3
*
(e) A1
3+
(c) Zn
2+
9. K 2 CrO 4 is slowly added to a solution that is 0.0200 M in Pb(NO 3 ) 2 and 0.0200 M
in Ba(NO 3 ) 2 .
(a) Which ion precipitates first?
(b) What will be its concentration when the second ion begins to precipitate?
10. A 1.00 ml sample of 0.100 M Pb(NO 3 ) 2 is added to 100 ml of a solution saturated with SrCrO 4 . How many milligrams of PbCrO 4 will precipitate?
11. What concentration of C 2 O
2 ~ is needed to start precipitation of calcium from a
saturated solution of CaSO 4 ?
12. An analyst wishes to determine the amount of calcium by precipitation as the
oxalate. If his sample contains 50.0 mg Ca
2+ in 250 ml of solution, and if he
adds (NH 4 ) 2 C 2 O 4 to give an oxalate ion concentration of 0.500 M, what percentage of his Ca
2+ will remain unprecipitated?
13. What will be the concentration of Cu
2+ remaining in solution when Cd
2+ just
begins to precipitate as CdS from a solution that was originally 0.0200 M in
both Cu
2+ and Cd
2+ ?
14. A solution is 0.0200 M in Mg
2+ and 0.100 M in NH 4 NO 3 . What concentration of
NH 3 must be attained in order to begin precipitation of Mg(OH) 2 ?
15. In order to prevent precipitation of Mg(OH) 2 , what is the minimal number of
grams of NH 4 C1 that must be added to 500 ml of a solution containing 3.00 g of
Mg(NO 3 ) 2 and 5.00 g of NH 3 ?
16. How many moles of SrF 2 will dissolve in 250 ml of 0.100 M HNO 3 ?
17. What is the pH of a water solution saturated with Fe(OH) 3 ?
18. (a) To what final pH must a solution be adjusted in order to precipitate as much
CdS as possible without precipitating any ZnS? (The solution is originally
0.0200 M with respect to each metal ion.)
(b) How much Cd
2+ will be left in solution when the Zn
2+ begins to precipitate?
19. What must be the final pH of an H 2 SO 4 solution in order to just dissolve 0.0500
mole of ZnS in 1 liter of the solution?
20. What is the lowest pH that will permit the sulfide of each of the following
metals to precipitate from a 1.00 x 10~
4 M solution saturated with H 2 S?
(a) Pb
2+
(c) Co+
(b) Bi
3+
(d) Mn
2+
21. Can a suspension of Agl that is 5.00 M in HC1 be converted to Ag 2 S by
saturation with H 2 S? Explain.
22. How many liters of a solution saturated with HgS would one have to take to
find, statistically, one Hg
2+ ion? (Neglect hydrolysis of the sulfide ion.)
385
(a) Cu
2+
(d) Sn
2+
(b) Cr
3
*
(e) A1
3+
(c) Zn
2+
9. K 2 CrO 4 is slowly added to a solution that is 0.0200 M in Pb(NO 3 ) 2 and 0.0200 M
in Ba(NO 3 ) 2 .
(a) Which ion precipitates first?
(b) What will be its concentration when the second ion begins to precipitate?
10. A 1.00 ml sample of 0.100 M Pb(NO 3 ) 2 is added to 100 ml of a solution saturated with SrCrO 4 . How many milligrams of PbCrO 4 will precipitate?
11. What concentration of C 2 O
2 ~ is needed to start precipitation of calcium from a
saturated solution of CaSO 4 ?
12. An analyst wishes to determine the amount of calcium by precipitation as the
oxalate. If his sample contains 50.0 mg Ca
2+ in 250 ml of solution, and if he
adds (NH 4 ) 2 C 2 O 4 to give an oxalate ion concentration of 0.500 M, what percentage of his Ca
2+ will remain unprecipitated?
13. What will be the concentration of Cu
2+ remaining in solution when Cd
2+ just
begins to precipitate as CdS from a solution that was originally 0.0200 M in
both Cu
2+ and Cd
2+ ?
14. A solution is 0.0200 M in Mg
2+ and 0.100 M in NH 4 NO 3 . What concentration of
NH 3 must be attained in order to begin precipitation of Mg(OH) 2 ?
15. In order to prevent precipitation of Mg(OH) 2 , what is the minimal number of
grams of NH 4 C1 that must be added to 500 ml of a solution containing 3.00 g of
Mg(NO 3 ) 2 and 5.00 g of NH 3 ?
16. How many moles of SrF 2 will dissolve in 250 ml of 0.100 M HNO 3 ?
17. What is the pH of a water solution saturated with Fe(OH) 3 ?
18. (a) To what final pH must a solution be adjusted in order to precipitate as much
CdS as possible without precipitating any ZnS? (The solution is originally
0.0200 M with respect to each metal ion.)
(b) How much Cd
2+ will be left in solution when the Zn
2+ begins to precipitate?
19. What must be the final pH of an H 2 SO 4 solution in order to just dissolve 0.0500
mole of ZnS in 1 liter of the solution?
20. What is the lowest pH that will permit the sulfide of each of the following
metals to precipitate from a 1.00 x 10~
4 M solution saturated with H 2 S?
(a) Pb
2+
(c) Co+
(b) Bi
3+
(d) Mn
2+
21. Can a suspension of Agl that is 5.00 M in HC1 be converted to Ag 2 S by
saturation with H 2 S? Explain.
22. How many liters of a solution saturated with HgS would one have to take to
find, statistically, one Hg
2+ ion? (Neglect hydrolysis of the sulfide ion.)