CEE 680

 

14 November 2006

SECOND EXAM

 

Closed book, two pages of notes allowed.

 

Answer all questions.  Please state any additional assumptions you made, and show all work.

 

 

1. Carbonate System.

(40% for both parts) You’re treating a drinking water that contains 25 mg/L of alkalinity.  The initial pH is 6.2, and you wish to raise this to 8.5 for purposes of corrosion control.

 

A.   (20%) How much of a caustic soda (NaOH) dose do you need to add to accomplish this if the water goes right into the water main?

B.   (20%) Answer the above question, but this time assume you have a large finished water reservoir after caustic addition that allows the water to reach equilibrium with the atmosphere before entering the distribution system.

 

 

 

2. Complexation

 

(40% for both parts) Aqueous cyanide forms strong complexes with many metals.  The following two part problem concerns complexes with cadmium and zinc.

 

A. (20%) On the attached blank graph template, sketch out a set of alpha curves (vs log[ CN- ]) for the Cadmium-cyanide system.  Point out they key graphical features that you used to make this sketch (e.g. intersection points).  Assume that the pH is high enough so that there is no significant formation of HCN.  Use the following stability constants from Benjamin’s text.  Note that these are all overall formation constants.

 

CdL

Log b1 = 5.32

CdL2

Log b2 = 10.37

CdL3

Log b3 = 14.83

CdL4

Log b4 = 18.29

 

 

B. (20%) Attached is an accurate graph of alpha values (vs log[ CN- ]) for the Zinc-cyanide system.    Using this graph determine the species composition when the total zinc concentration is 0.1 mM and the total cyanide concentration is 0.5 mM.  Assume once again that the pH is high enough so that there is no significant formation of HCN.

 

 

 

3. Multiple Choice. 

(20%) Answer all 10 of the following questions.  Indicate which of the options is the best choice.

 

               1. The sum of total acidity and total alkalinity on any given sample is equal to

a. the UV absorbance

                           b. twice the total carbonate

                           c. the value one

                           d. half of the hardness

                           e. zero

 

               2. When a solution spontaneously absorbs CO2 from the atmosphere it:

                           a. resuts in higher total carbonate

                           b. drops in pH

                           c. approaches equilibrium

                           d. all of the above

                           e. none of the above

 

               3. Phenolphthalein

                           a. is a hexadentate ligand

                           b. is rarely used because noone can spell it

                           c. complexes calcium forming an insoluble salt

                           d. is the drug of choice for malaria

e. changes from colorless to red as pH increases

 

               4. H2CO3*:

                           a. is composed mostly of aqueous CO2

                           b. is conservative in closed systems

c. is am ampholyte

                           d. all of the above

                           e. none of the above

 

               5. Ion pairs:

                           a. are always charged

                           b. are larger than Bartlett pears

                           c. are almost impossible to separate

                           d. are outer-sphere complexes

 

               6. The coordination number:

                           a. is usually 6 or less

                           b. is related to the charge on the central atom

                           c. is a function of the size of the ligand

                           d. all of the above

                           e. none fo the obove

 

               7. The buffer intensity of the open carbonate system:

                           a. is independent of the alkalinity

                           b. is independent of the CT

                           c. is always higher than the pCO2

                           d. is at a minimum where the pH = pK1

                           e. is at a minimum where the pH = pK2

 

               8. Detergent “builders” are used to:

                           a. help solubilize grease

                           b. complex trace metals

                           c. take hardness cations from the surfactants

                           d. elevate the acidity

                           e. reduce the caloric content

 

               9. EDTA

                           a. stands for ethylene dinitro tetraacetic acid

                           b. is most commonly used as a pH buffer

                           c. is a higly potent carcinogen

                           d. all of the above

                           e. none of the above

 

               10. The Irving Williams Series

                           a. is a means of estimating alkalinity

                           b. describes the inverse proportionality of acidity to alkalinity

                           c. includes a number of books, such as The Chapman Report, and The Prize

                           d. provides a comprehensive description of ligand structure

                           e. follows the increase in ligand affinity from Mn(II) to Cu(II)

 

 

Selected Acidity Constants  (Aqueous Solution, 25°C, I = 0)

   NAME

   FORMULA

 pKa

Perchloric acid

HClO4 = H+ + ClO4-

-7         STRONG

Hydrochloric acid

HCl = H+ + Cl-

-3

Sulfuric acid

H2SO4= H+ + HSO4-

-3  (&2)    ACIDS

Nitric acid

HNO3 = H+ + NO3-

-0               

Hydronium ion

H3O+ = H+ + H2O

 0               

Trichloroacetic acid

CCl3COOH = H+  + CCl3COO-

 0.70

Iodic acid

HIO3 = H+ + IO3-

 0.8

Bisulfate ion

HSO4- = H+ + SO4-2

 2

Phosphoric acid

H3PO4 = H+ + H2PO4-

 2.15 (&7.2,12.3)

Citric acid

C3H5O(COOH)3= H+  + C3H5O(COOH)2COO-

 3.14 (&4.77,6.4)

Hydrofluoric acid

HF = H+  + F-

 3.2

m-Hydroxybenzoic acid

C6H4(OH)COOH = H+  + C6H4(OH)COO-

 4.06  (&9.92)

p-Hydroxybenzoic acid

C6H4(OH)COOH = H+  + C6H4(OH)COO-

 4.48  (&9.32)

Nitrous acid

HNO2 = H+  + NO2-

 4.5

Acetic acid

CH3COOH = H+  + CH3COO-

 4.75

Propionic acid

C2H5COOH = H+  + C2H5COO-

 4.87

Carbonic acid

H2CO3 = H+  + HCO3-

 6.35 (&10.33)

Hydrogen sulfide

H2S = H+  + HS-

 7.02 (&13.9)

Dihydrogen phosphate

H2PO4- = H+  + HPO4-2

 7.2

Hypochlorous acid

HOCl = H+  + OCl-

 7.5

Boric acid

B(OH)3 + H2O = H+  + B(OH)4-

 9.2 (&12.7,13.8)

Ammonium ion

NH4+ = H+  + NH3

 9.24

Hydrocyanic acid

HCN = H+  + CN-

 9.3

p-Hydroxybenzoic acid

C6H4(OH)COO-  = H+  + C6H4(O)COO-2

 9.32

Phenol

C6H5OH = H+  + C6H5O-

 9.9

m-Hydroxybenzoic acid

C6H4(OH)COO-  = H+  + C6H4(O)COO-2

 9.92

Bicarbonate ion

HCO3- = H+  + CO3-2

10.33

Monohydrogen phosphate

HPO4-2  = H+  + PO4-3

12.3

Bisulfide ion

HS-  = H+  + S-2

13.9          

Water

H2O = H+  + OH-

14.00         

Methane

CH4 = H+ + CH3-

34

 

Problem 2A

 

Problem 2B