Worksheet : Ch. 15 : Test Reviewdate

AP ChemistryName :

Worksheet : Ch. 15 : Test ReviewDate :

Questions 1–4 refer to aqueous solutions containing 1:1 mole ratios of the following pairs of substances. Assume all concentrations are 1 M.

(A)NH3 and NH4Cl

(B)H3PO4 and NaH2PO4

(C)HCl and NaCl

(D)NaOH and NH3

(E)NH3 and HC2H3O2 (acetic acid)

_____ 1. The solution with the lowest pH

_____ 2. The most nearly neutral solution

_____ 3. A buffer at a pH > 8

_____ 4. A buffer at a pH < 6

_____ 5. The formula for the hexaaquochromium(III) ion is

a. [Cr(H2O)6]2+ b. [Cr2(H2O)6]3+ c. [Cr3(H2O)6]3+ d. [Cr(H2O)6]3+ e. [Cr(H2O)8]3+

…CN- + …Ag + …. O2 + ….H2O  ….Ag(CN) 2- + ….OH-

_____ 6. The number of OH- ions in the above balanced reaction is :

a. 6 b. 5 c. 4 d. 3 e. 2

_____ 7. What is the [Ag+] in a saturated solution of silver chromate if the concentration of the chromate ion is 1.0 x 10-4 M (Ksp for silver chromate is 9.0 x 10-12).

a. 3.0 x10-8 b. 9.0 x10-8 c. 3.0 x10-4 d. 1.0 x10-4 e. 9.0 x10-2

acid / Ka (mol dm-3)
hydrofluoric acid / 5.6 x 10-4
methanoic acid / 1.6 x 10-4
ethanoic acid / 1.7 x 10-5
hydrogen sulphide / 8.9 x 10-8

_____ 8. Given the Ka values on the above table, which of the following acids would be preferred to make a buffer with a neutral pH?

a. hydrofluoric b. methanoic c. ethanoic d. hydrogen sulphide

_____ 9. Which of the following occurs when excess concentrated NH3(aq) is mixed thoroughly with 0.1 M Cu(NO3)2(aq)?

(a) A dark red precipitate forms and settles out.

(b) Separate layers of immiscible liquids form with a blue layer on top.

(d) Bubbles of ammonia gas form.

(e) The pH of the solution decreases

_____ 10. When 100 mL of 1.0 M Na3PO4 is mixed with 100 mL of 1.0 AgNO3, a yellow precipitate forms and [Ag+] becomes negligibly small. Which of the following is a correct listing of the ions remaining in solution in order of increasing concentration?

(a) [PO43-] < [NO3-] < [Na+]

(b) [PO43-] < [Na+] < [NO3-]

(d) [Na+] < [NO3-] < [PO43-]

(e) [Na+] < [PO43-] < [NO3-]

11. Write equations for the stepwise formation of each of the following complex ions.

a. Co(NH3)62+

12. Concentrated ammonia is added to an aqueous solution of copper(II) sulfate. Initially, a white precipitate forms. As more ammonia is added, the precipitate dissolves and the solution becomes a deep bluish purple. Write equations describing the reactions that are occurring. [Hint: Cu2+reacts with NH3 to form Cu(NH3)42+.]

13. The overall formation constant for HgI42- is 1.0 x 1030. That is,

What is the concentration of Hg2+ in 500.0 mL of a solution that was originally 0.010 M Hg2+ and 0.78 M I-? The reaction is Hg2+ + 4 I- ↔ HgI42- (Hint : Assume reaction goes to completion first).

14. The copper(I) ion forms a chloride salt that has Ksp= 1.2 x 10-6. Copper(I) also forms a complex ion with Cl-: Cu+ + 2 Cl- ↔ CuCl2- K = 8.7 x 104

a. Calculate the solubility of copper(I) chloride in pure water.

b. Calculate the solubility of copper(I) chloride in 0.10 MNaCl. (Hint : You need to write two chemical equations to get the equation described in this question and then determine the K value for the added equations).

15. Silver(I) chloride dissolves readily in 2 M NH3 but is quiteinsoluble in 2 M NH4NO3. Explain.

16. a. Calculate the pH of a buffered solution that is 0.100 M in C6H5CO2H (benzoic acid, Ka = 6.4 x 10-5) and 0.100 M in C6H5CO2Na.

b. Calculate the pH after 20.0% (by moles) of the benzoic acid is converted to benzoate anion by addition of base. Use the dissociation equilibrium

C6H5CO2H ↔ C6H,CO2- + H+

to calculate the pH.

c. Do the same as in part b, but use the following equilibrium to calculate the pH:

C6H5CO2- + H2O ↔ C6H5CO2H + OH-

d. Do your answers in parts b and c agree? Explain.

17. An aqueous solution contains dissolved NH4C1 and NH3. The concentration of NH3(KbNH3 = 1.8 x 10–5)is 0.500 M, and the pH is 8.95.

a. Calculate the equilibrium concentration of NH4+.

b. Calculate the pH after 4.00 g of NaOH(s) is added to 1.00 L of this solution. (Neglect any volume changes.)

18. Tris(hydroxymethyl)aminomethane, commonly called TRIS or Trizma, is often used as a buffer in biochemical studies. Its buffering range is pH 7 to 9, and Kbis 1.19 x 10-6 for the reaction

(HOCH2)3CNH2 + H2O ↔ (HOCH2)3CNH3+ + OH-
TRISTRISH+

a. What is the optimal pH for TRIS buffers?

b. Calculate the ratio [TRIS]/[TRISH+] at pH = 7.00 and at pH = 9.00.

c. A buffer is prepared by diluting 50.0 g TRIS base and 65.0 g TRIS hydrochloride (written as TRISHC1) to a total volume of 2.0 L. What is the pH of this buffer? What is the pH after 0.50 mL of 12 M HC1 is added to a 200.0-mL portion of the buffer?

19. Potassium hydrogen phthalate, known as KHP (molar mass = 204.22 g/mol), can be obtained in high purity and is used to determine the concentration of solutions of strong bases by the reaction

HP- +OH-  H2O + P2-

If a typical titration experiment begins with about 0.5 g KHP and has a final volume of about 100 mL, what would be an appropriate indicator to use? The pKa for HP- is 5.51.

20. The Kspof hydroxyapatite(tooth enamel), Ca5(PO4)3OH, is 6.8 x l0-37. Calculate the solubility of hydroxyapatite in pure water in moles per liter. How is the solubility of hydroxyapatite affected by adding acid? When hydroxyapatite is treated with fluoride, the mineral fluoroapatite, Ca5(PO4)3F, forms. The Ksp of this substance is 1 x 10-60. Calculate the solubility of fluoroapatite in water. How do these calculations provide a rationale for the fluoridation of drinking water?

21. A 10.00 milliliter sample of methylamine, CH3NH2, solution is titrated with a standard HCl solution.

a. _____ (T/F) An error will be introduced if distilled water is added to the 10.00 mL sample of methylamine.

b. _____ (T/F) An error will be introduced if distilled water is added to the standard HCl.

c. _____ (T/F) Adding water to the standard will cause the calculated molarity of the methylamine to be too high.

d. _____ (T/F) An indicator which changed color at a pH below 7 would be appropriate for this toitration.

e. _____ (T/F) An indicator which changed color above 7 would yield calculations which would show the molarity of the methylamine to be too high.

Answers : 1. c 2. e 3. a 4. b 5. d 6. c 7. c 8. d 9. c 10. a 11. CO2+ + NH3 ↔ CoNH32+ K1 ; CONH32+ + NH3 ↔ Co(NH3)22+ K2 ; Co(NH3)22+ + NH3 ↔ Co(NH3)32+ K3 …. Overall : CO2+ + 6NH3 ↔ Co(NH3)62+ K = K1K2K3K4K5K6 12. Cu2+ + 2 OH-  Cu(OH)2(s) ; Cu(OH)2(s) + 4 NH3 Cu(NH3)42+(aq) + 2OH-(aq) 13. 3.3 x 10-32 M 14a. 1.1 x 10-3 M 14b. 9.1 x 10-3 M 15. In 2.0 M NaOH, the soluble complex (Ag(NH3)2+ forms which increases the solubility of AgCl. The NH4+ of NH4OH is a weak acid and does not provide enough NH3 to complex with the Ag+ ions. 16a. 4.19 16b. 4.37 16c. 4.37 16d. Yes, Kb and Ka are related by Kw and [H+] and [OH-] are related by Kw. Both equilibria must hold true given the same concentrations. 17a. 1.0 M 17b. 9.1 18a. 8.076 18b. for pH = 7.00, ratio is 0.083, for pH = 9.00, ratio = 8.3 18c. initial pH = 8.08, after acid is added : 7.95 19. pH = 8.9, phenolphthalein would be a good indicator 20. 2.7 x 10-5 M ; the solubility of hydroxyapatite will increases a solution gets more acidic, since both the phosphate and the hydroxide can react with H+ ions. 6 x 10-8 M; the hydroxyapatite is converted to the less soluble fluoroapatite by fluoride ions in drinking water. The less soluble fluoroapatite will then be more difficult to dissolve making teeth less susceptible to decay. 21 a. F (does not change no. of moles) b. T c. T ( moles methalymine are calc from moles of HCl (which are calc from L x M of acid)) d. T e. F (too low vol. of HCl needed)