CEE 680

7 October 1999

FIRST EXAM

 

Closed book, one page of notes allowed.

Answer all questions. Please state any additional assumptions you made, and show all work. You are welcome to use a graphical method of solution if it is appropriate.

 

Miscellaneous Information:

R = 1.987 cal/mole° K = 8.314 J/mole°K

Absolute zero = -273.15° C

1 joule = 0.239 calories

1015 newtons = 1 figanewton

 

 

 

  1. (50%) What is the pH of a 0.05 F solution of Sodium Sulfide (Na2S) to which you have added 0.09 M of HCl ? (Ignore ionic strength effects)

a. 25° C

b. 100° C.

Preferred Approach

a.

 

b.

determine enthalpy change for the reaction:

H2S = H+ + HS-

then re-estimate Ka

and now:

 

Alternative Approach #1

([Na+] + [Cl-]) + [H+] = [OH-] + [HS-] + 2[S-2]

 

 

Alternative Approach #2

2[H2S] + [HS-] + [H+] = [OH-] + [Cl-]

 

Note: this approach doesn’t work very well. It’s hard to find the intersection with any accuracy. This is the general problem one encounters in using the PBE with solutions to which one has added a strong acid or base.

 

 

2. (40%) What is the complete composition of a 1 liter volume of water to which you have added 10-3 M of ammonium nitrate (NH4NO3) and 10-4 M of disodium hydrogen phosphate (Na2HPO4) ? Approximate values (± 0.2 log units) will suffice.

 

Approach

 

the PBE is:

2[H3PO4] +[H2PO4-] + [H+] = [OH-] + [NH3] + [PO4-3]

from the graph below, we can conclude that the solution lies at:

[H2PO4-] = [NH3]

 

 

pH =7.7

log [H3PO4] = -10.1

log [H2PO4-] = -4.6

log [HPO4-2] = -4.2

log [PO4-3] = -8.8

log [OH-] = -6.3

log [NH3] = -4.6

log [NH4+] = -3

or:

[H+] = 2x10-8

log [H3PO4] = 8x10-11

log [H2PO4-] = 2.5x10-5

log [HPO4-2] = 6x10-5

log [PO4-3] = 1.6x10-9

log [OH-] = 5x10-7

log [NH3] = 2.5x10-5

log [NH4+] = 10-3

 

 

 

3. (10%) True/False. Mark each one of the following statements with either a "T" or an "F".

a. F The pH of a solution of a strong acid is always equal to its pC.

b. T Free protons generally don’t exist in aqueous solution.

c. T Buffer intensity is a measure of the degree to which a solution resists change in pH

d. F A solution with a very high acid neutralizing capacity will necessarily have a very high buffer intensity as well.

e. F Reactions with a large negative D G will proceed very quickly toward the right.

f. T Acetic Acid is not a strong acid.

g. F Strong acids have strong conjugate bases.

h. F The pH of a solution prepared from an equimolar concentration of a weak acid and its conjugate base is approximately equal to the pC.

i. F One can usually assume that the protonated form (i.e., acid form) is negligible compared to the deprotonated form when a salt (i.e., conjugate base) of a simple monoprotic acid is added to water.

j. F The value of a o can only exceed a value of 1.00 at pH’s far below the pKa.

 

 

 

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)

o-Phthalic acid

C6H4(COOH)2 = H+ + C6H4(COOH)COO-

2.89 (&5.51)

Citric acid

C3H5O(COOH)3= H+ + C3H5O(COOH)2COO-

3.14 (&4.77,6.4)

Hydrofluoric acid

HF = H+ + F-

3.2

Aspartic acid

C2H6N(COOH)2= H+ + C2H6N(COOH)COO-

3.86 (&9.82)

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

Ammonia

NH3 = H+ + NH2-

23

Methane

CH4 = H+ + CH3-

34

Species

kcal/mole

kcal/mole

Ca+2(aq)

-129.77

-132.18

CaC03(s), calcite

-288.45

-269.78

CaO (s)

-151.9

-144.4

C(s), graphite

0

0

CO2(g)

-94.05

-94.26

CO2(aq)

-98.69

-92.31

CH4 (g)

-17.889

-12.140

H2CO3 (aq)

-167.0

-149.00

HCO3- (aq)

-165.18

-140.31

CO3-2 (aq)

-161.63

-126.22

CH3COO-, acetate

-116.84

-89.0

H+ (aq)

0

0

H2 (g)

0

0

Fe+2 (aq)

-21.0

-20.30

Fe+3 (aq)

-11.4

-2.52

Fe(OH)3 (s)

-197.0

-166.0

Mn+2 (aq)

-53.3

-54.4

MnO2 (s)

-124.2

-111.1

Mg+2 (aq)

-110.41

-108.99

Mg(OH)2 (s)

-221.00

-199.27

NO3- (aq)

-49.372

-26.43

NH3 (g)

-11.04

-3.976

NH3 (aq)

-19.32

-6.37

NH4+ (aq)

-31.74

-19.00

HNO3 (aq)

-49.372

-26.41

O2 (aq)

-3.9

3.93

O2 (g)

0

0

OH- (aq)

-54.957

-37.595

H2O (g)

-57.7979

-54.6357

H2O (l)

-68.3174

-56.690

SO4-2

-216.90

-177.34

HS- (aq)

-4.22

3.01

H2S(g)

-4.815

-7.892

H2S(aq)

-9.4

-6.54