CEE 680

 

20 November 2008

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.

 

(55% for both parts) Two raw drinking waters are mixed as they enter the headworks of a water treatment plant. The two are characterized as follows:

 

Water

Flow (MGD)

Alkalinity

(mg/L as CaCO3)

pH

#1

20

25

6.50

#2

10

300

8.85

 

A. What will the pH of the blended water be immediately after mixing?

 

B. What will the pH of the blended water be after it has reached equilibrium with the bulk atmosphere?

 

C. How many mg/L of caustic soda (NaOH) must be added to the unequilibrated blended water in part A to raise the pH to 9.80 ?

 

 

 

2. Complexation

 

(45% for both parts) Aqueous fluoride forms strong complexes with many metals. The following two part problem concerns complexes with Beryllium.

 

A.    (20%) Attached is an accurate graph of alpha values (vs log[ F-]) for the Beryllium-Fluoride system (equilibria data shown below). Using this graph determine the complete species composition when the total Beryllium concentration is 0.10 mM and the total fluoride concentration is 0.40 mM. Ignore the possible formation of any other complexes other than those from Be and F; also ignore any possible precipitation reactions. Assume the water is a neutral pH.

 

 

 

 

 

 

 

B.    (20%) Describe in qualitative terms the impacts of pH on this system. Estimate in quantitative terms what the concentrations of each of the species would be if the pH were 2.2, instead of 7.0, and explain how you got these.

 

 

 


Selected Acidity Constants (Aqueous Solution, 25C, 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

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

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-

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