Problems B
399
24. A 100 ml sample of a solution containing 10.0 mg Cu
2+ is treated with 50.0 ml
of 3.00 M NH 3 . Calculate the Cu
2+ concentration in the resulting solution.
25. What is the minimum concentration of CN~ that will prevent the precipitation
of Hg(OH) 2 from a solution that is 0.500 M in OH~ and 0.100 M in Hg
2+ ? [K sp
for Hg(OH) 2 is 2.00 x 10~
22 .]
26. What volume of 6.00 M NH 3 must be added to 250 ml of 0.100 M AgNO 3 to
prevent the precipitation of AgCl when 4.00 g of NaCl are added?
27. Calculate the pH of (a) a 0.100 M Mg(NO 3 ) 2 solution, and (b) a 0.100 M
Hg(NO 3 ) 2 solution.
28. Answer the following questions involving the typical amphoteric hydroxides,
Zn(OH) 2 and Cr(OH) 3 .
(a) Calculate the Zn
2+ , Zn(OH)|~, and OH~ concentrations in a water solution
saturated with Zn(OH) 2 .
(b) Calculate the Cr'
)+ and Cr(OH)^ concentrations that result when an excess
of solid Cr(OH) 3 is added to 500 ml of 1.50 M NaOH.
(c) A saturated Zn(OH) 2 solution is shown to be 0.250 M in NaOH. Calculate
the concentrations of Zn
2+ and Zn(OH)f~ ions.
(d) An excess of solid Zn(OH) 2 is added to 100 ml of 1.00 M NaOH. How
many moles of Zn(OH) 2 are dissolved?
(e) Compare (;') the minimal volume of 1.00 M NH 3 required to dissolve 0.100
mole of Zn(OH) 2 to give Zn(NH 3 )f
+ with (iV) the minimal volume of 1.00 M
NaOH required to dissolve 0.100 mole of Zn(OH) 2 to give Zn(OH)
2 ,-.
29. It is proposed, by treatment with the minimal amount of NH 3 necessary to
dissolve the AgCl, to separate a solid mixture of 0.500 g of AgCl and 0.750g of
AgBr.
(a) What must be the NH 3 concentration in the final reaction mixture if all the
AgCl is dissolved in a total volume of 200 ml?
(b) What volume of 1.00 M NH 3 should be added to the solids to just dissolve
the AgCl?
(c) How many grams of AgBr would also dissolve if the separation were
performed as in (b)?
30. A 2.0000 g mixture of solids containing NaNO 3 , NaCl, and NaBr is dissolved
in water, and the Cl~ and Br~ are precipitated with an excess of AgNO 3 . This
silver halide precipitate, which weighs 2.5000 g, is treated with 200 ml of
0.900 M NH 3 that dissolves the AgCl. The remaining residue of AgBr weighs
0.9000 g.
(a) Calculate the percentages of NaCl and NaBr in the original mixture.
(b) By what percentage are the analyses in (a) in error as a result of the
solution of some AgBr by the NH 3 ?
399
24. A 100 ml sample of a solution containing 10.0 mg Cu
2+ is treated with 50.0 ml
of 3.00 M NH 3 . Calculate the Cu
2+ concentration in the resulting solution.
25. What is the minimum concentration of CN~ that will prevent the precipitation
of Hg(OH) 2 from a solution that is 0.500 M in OH~ and 0.100 M in Hg
2+ ? [K sp
for Hg(OH) 2 is 2.00 x 10~
22 .]
26. What volume of 6.00 M NH 3 must be added to 250 ml of 0.100 M AgNO 3 to
prevent the precipitation of AgCl when 4.00 g of NaCl are added?
27. Calculate the pH of (a) a 0.100 M Mg(NO 3 ) 2 solution, and (b) a 0.100 M
Hg(NO 3 ) 2 solution.
28. Answer the following questions involving the typical amphoteric hydroxides,
Zn(OH) 2 and Cr(OH) 3 .
(a) Calculate the Zn
2+ , Zn(OH)|~, and OH~ concentrations in a water solution
saturated with Zn(OH) 2 .
(b) Calculate the Cr'
)+ and Cr(OH)^ concentrations that result when an excess
of solid Cr(OH) 3 is added to 500 ml of 1.50 M NaOH.
(c) A saturated Zn(OH) 2 solution is shown to be 0.250 M in NaOH. Calculate
the concentrations of Zn
2+ and Zn(OH)f~ ions.
(d) An excess of solid Zn(OH) 2 is added to 100 ml of 1.00 M NaOH. How
many moles of Zn(OH) 2 are dissolved?
(e) Compare (;') the minimal volume of 1.00 M NH 3 required to dissolve 0.100
mole of Zn(OH) 2 to give Zn(NH 3 )f
+ with (iV) the minimal volume of 1.00 M
NaOH required to dissolve 0.100 mole of Zn(OH) 2 to give Zn(OH)
2 ,-.
29. It is proposed, by treatment with the minimal amount of NH 3 necessary to
dissolve the AgCl, to separate a solid mixture of 0.500 g of AgCl and 0.750g of
AgBr.
(a) What must be the NH 3 concentration in the final reaction mixture if all the
AgCl is dissolved in a total volume of 200 ml?
(b) What volume of 1.00 M NH 3 should be added to the solids to just dissolve
the AgCl?
(c) How many grams of AgBr would also dissolve if the separation were
performed as in (b)?
30. A 2.0000 g mixture of solids containing NaNO 3 , NaCl, and NaBr is dissolved
in water, and the Cl~ and Br~ are precipitated with an excess of AgNO 3 . This
silver halide precipitate, which weighs 2.5000 g, is treated with 200 ml of
0.900 M NH 3 that dissolves the AgCl. The remaining residue of AgBr weighs
0.9000 g.
(a) Calculate the percentages of NaCl and NaBr in the original mixture.
(b) By what percentage are the analyses in (a) in error as a result of the
solution of some AgBr by the NH 3 ?