# Extra Problems for Final

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## Exercises

### Stoichiometry

1. A student was given 1.00 g of ammonium dichromate for the preparation of a coordination compound. The sample was ignited, thereby producing chromium(III) oxide, water, and nitrogen gas. The chromium(III) oxide was allowed to react at 600°C with carbon tetrachloride to yield chromium(III) chloride and phosgene (COCI2). Upon treatment with excess liquid ammonia, the chromium(III) chloride reacted to produce hexaamminechromium(III) chloride. Calculate the maximum amount of hexaamminechromium(III) chloride that the student could prepare from the 1.00-g sample of ammonium dichromate.

2. When silver nitrate is added to a solution of a substance with the empirical formula CoCl3 · 5NH3, how many moles of AgCl will be precipitated per mole of cobalt present? Why?

3. Co(III) occurs in octahedral complexes with the general empirical formula CoClm · nNH3. What values of n and m are possible? What are the values of n and m for the complex that precipitates 1 mole of AgCl for every mole of Co present?

4. How many ions per mole will you expect to find in solution when a compound with the empirical formula PtCl4- · 3NH3 is dissolved in water? What about PtCl2 · 3NH3? Draw diagrams of each of the complex cations.

5. Each of the following is dissolved in water to make a 0.001 M solution. Rank the compounds in order of decreasing conductivity of their solutions; K2PtCl6, Co(NH3)6Cl3, Cr(NH3)4Cl3, Pt(NH3)6Cl4. Rewrite each compound by using brackets to distinguish the complex ion present in aqueous solution.

### Formulas and nomenclature

6. Give the systematic names of [Co(NH3)4Cl2]Br, K3Cr(CN)5, and Na2CoCl4.

7. Write the formulas for each of the following compounds by using brackets to distinguish the complex ion from the other ions: (

1. hexaaquonickel(II) perchlorate
2. trichlorotriammineplatinum(IV) bromide
3. dichlorotetraammineplatinum(IV) sulfate
4. potassium monochloropentacyanoferrate (III)

8. Write the formula for each of the following by using brackets to distinguish the complex ion:

1. hydroxopentaaquoaluminum(III) chloride
2. sodium tricarbonatocobaltate(III)
3. sodium hexacyanoferrate(II)
4. ammonium hexanitrocobaltate(III)

### Isomers

9. How many isomers are there of the compound [Cr(NH3)4Cl2]Cl? Sketch them.

10. Sketch all the geometrical and optical isomers of PtCl2l2(NH3)2.

11. How many geometrical and optical isomers are there of the complex ion Co(en)2Cl;? Of these, how many pairs of isomers are there differing only by a mirror reflection? How many isomers have a plane of symmetry and hence do not exist in pairs of optical isomers?

12. Repeat Problem 11 with propylenediamine substituted for ethylenediamine. Ignore optical isomers from the propylene carbon.

13. How many different structural isomers are there of a substance with the empirical formula FeBrCl · 3NH3 · 2H2O? For each different structural isomer, how many different geometrical isomers exist? How many of these can be grouped into right-handed and left-handed pairs of optical isomers?

### Electronic structure

14. The Co2+ ion in aqueous solution is octahedrally coordinated and paramagnetic, with three unpaired electrons. Which one or ones of the following statements follow from this observation:

1. Co(H2O)42+ is square planar
2. Co(H2O)42+ is tetrahedral
3. Co(H2O)42+ has a Δ0 that is larger than the electron—pairing energy;
4. the d levels are split in energy and filled as follows: (t2g)5(eg)2
5. the d levels are split in energy and filled as follows: (t2g)6(eg)1

15. The coordination compound potassium hexafluorochromate(III) is paramagnetic. What is the formula for this compound? What is the configuration of the Cr d electrons?

16. How many unpaired electrons are there in Cr3+, Cr2+, Mn2+, Fe2+, Co3+, Co2+ in

1. a strong octahedral ligand Held and
2. a very weak octahedral field?

17. A low-spin tetrahedral complex has never been reported, although numerous high-spin complexes of this geometry have been prepared. What conclusion may be drawn regarding the magnitude of Δt from this fact?

18. Certain platinum complexes have been found to be active antitumor agents. Among these are cis-Pt(NH3)2Cl4, cis-Pt(NH3)2Cl2, and cis-Pt(en)Cl2 (none of the trans isomers are effective). Use valence bond theory to account for the diamagnetism of these complexes. Are these inner or outer complexes? What kinds of hybrid orbitals are used in bonding?

19. What is the d-orbital electronic configuration of Cr(NH3)63+? How many unpaired electrons are present? If six Br- groups were substituted for the six NH3 groups to give CrBr63-, would you expect Δ0 to increase or decrease?

20. Diagram the electronic arrangements in Fe(H2O)62+ and Fe(CN)64- for both the valence bond and crystal Held models. Briefly compare these models.

21. For each of the following, sketch the d-orbita1 energy levels and the distribution of d electrons among them:

1. Ni(CN)42- (square planar)
2. Ti(H2O)62+ (octahedral)
3. NiCl42- (tetrahedra1)
4. CoF63- (high-spin complex)
5. Co(NH3)63+ (low—spin complex)

22. Co(III) can occur in the complex ion Co(NH3)63+.

1. What is the geometry of this ion? ln the valence bond theory, what Co orbitals are used in making bonds to the ligands?
2. What is the systematic name for the chloride salt of this ion?
3. Using crystal Field theory, draw two possible d~electron configurations for this ion. Assign to them the labels high spin, low spin, paramagnetic, diamagnetic. Which two labels are correct for the ammine complex?
4. Co(NH3)63+ can be reduced to C.o(NH3)62+ by adding an electron. Draw the preferred d-electron configuration for this reduced ion. Why is it preferred?

23. Pt(II) can occur in the complex ion PtCl42-.

1. What is the geometry of this ion? In the valence bond theory, what Pt orbitals are used in making bonds to the Cl- ions?
2. What is the systematic name for the sodium salt of this ion?
3. Using crystal field theory, draw the d-electron configuration for this ion. ls the ion paramagnetic or diamagnetic?
4. Pt(II) can be oxidized to Pt(lV). Draw the d-electron configuration for the chloride complex ion of Pt(lV). Explain the difference between this configuration and that of Pt(II), Is the Pt(IV) chloride complex ion paramagnetic or diamagnetic?

### Formation constants

24. A solution is prepared that is 0.025M in tetraamminecopper(IIl), Cu(NH3)44+. What will be the concentration of Cu2+ hydrated copper ion if the ammonia concentration is 0.10, 0.50, 1.00, and 3.00 M respectively? What ammonia concentration is needed to keep the Cu2+ concentration less than 10-15 M?

25. From the data in Table 20-8, calculate the pH of a 0.10M solution of Cr3+ ion. Hint: Consider the reactions

Cr3+ + H2O ↔ Cr(OH)2+ + H+ K = ?
Cr3+ + OH- ↔ Cr(OH)2+ Kf = 1 x 1010
H+ + OH- ↔. H2O Kw= ?

26. From the data in Table 20-8, calculate the pH of 0.10M solutions of Mn2+, Fe2+, and Ag+. See Problem 25 if you need help. From the results of these two problems, can you correlate the “acidity” of positive ions with their charge?

27. The ion Co(NH3)63+ is very stable, with Kf = 2.3 x 1034. If the hydrolysis constant for the ammonium ion, Kb, is 5 x 10-10, show that the equilibrium in the reaction

Co(NH3)63+ + 6H+ ↔ Co3+ + 6NH4+

lies far to the right. Then why does Co(NH3)63+ remain intact in hot concentrated sulfuric acid?

28. What is the concentration of chromate ion, CrO42- when solid BaCrO4 is placed in contact with water? What is the chromate ion concentration when solid BaCrO4 is placed in contact with a solution of 0.2M Ba2+? BaCrO4 can be dissolved in a solution of pyridine (py), producing the complex Ba(py)22+, with a formation constant of 4 x 10-12, If 0.10M BaCrO4 is dissolved in a solution with a constant pyridine concentration of 1.0 mole liter-1, what is the concentration of Ba2+ ion?

29. What is the solubility of Cu(OH)2 in pure water? In buffer at pH 6? Copper(II) forms a complex with NH3, Cu(NH3)42+, with Kf = 1.0 x 1012. What concentration of ammonia must be maintained in a solution to dissolve 0.10 mole of Cu(OH)2 per liter of solution?

30. Calculate the silver ion concentration in a saturated solution of AgCl in water. Silver ions react with an excess of Cl- as follows:

Ag+ + 2Cl- ↔ AgCl2 Kf : 1 x 102

Calculate the concentration of AgCl; and show that you were justified in ignoring the complex ion formation in calculating the silver ion concentration at the beginning of the problem.

31. The formation constant for the pyridine complex of silver

Ag+ + 2py ↔ 2 Ag(py)2+

is Kf : 1 x 1010. If a solution is initially 0.10M in AgNO3 and 1.0M in pyridine, what are the equilibrium concentrations of silver ion, pyridine, and the complex ion?

32. ln 0.10M NaCl, the concentration of silver ions cannot exceed 10-9 mole liter-1 because AgCl is so slightly soluble. What concentration of pyridine must be added to dissolve 0.10 mole of AgCl per liter of solution?

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