ENCE 433 Dr. Alba Torrents
EXAMINATION 2 "SHOW ALL WORK" Fall 1996
Thursday, October 31
No Partial credit for YES/NO; HIGH/LOW without explanation.
State assumption; remember to check them
Clearly indicate intermediate and final results
1. (25) Complete the following table to show how CT and ACIDITY
change when NaOH or HCl or CO2 etc. is added to an aqueous solution
in a closed system.
indicates the term increases.
--- indicates the term remains the same.
¯ indicates the term decreases.
|
CT |
ACIDITY |
|
|
NaOH |
||
|
HCl |
||
|
CO2 |
||
|
NaHCO3 |
||
|
Na2CO3 |
||
|
NaCl |
||
|
NH3 |
Recall: NH4+ NH3 + H+ pKa
= 9.3
2.(15) Determine the alkalinity of the following systems:
Assume that NH4Cl is a neutral salt and that HPO4-2
is the dominant species for the phosphate system
3. (30) Wastewater having CT = 2.0 x 10-3 M and
alkalinity = 1.0 x 10-3 M is discharged to lakewater having
CT = 2.5 x 10-3 M and alkalinity = 2.5 x 10-3
M.
(a) What are the CT and Alk of a 1:10 dilution (1 part wastewater,
9 parts lakewater) if the mixture is "closed" (i.e., not in equilibrium
with the atmosphere)? What is the pH?
(b) What concentration of strong acid or base would need to be added
to take this sample to its Na2CO3 equivalence point?
(assume closed system)
4. (30) Budget cuts have left a scientist with a sparsely stocked laboratory.
Besides pure water, sodium bicarbonate (NaHCO3) and sodium carbonate
(Na2CO3) are the only chemicals available.
(a) Describe how a solution having pH 9.0 can be prepared if no interaction
is allowed to take place with the atmosphere. (i.e., how many moles of
each chemical should be added to water to make 1 liter of pH 9.0 solution?)
ADDITIONAL INFORMATION
R = 1,987 cal /mole oK = 0.082 liter-atm deg-1 mole-1 = 8.31 joules deg-1 mole-1
PV = nRT
gal = 3.785 liters
F = 96,500 coulombs
2.3 RT/F = 0.059 V at 25oC
P(CO2) = 10-3.5 atm; P (O2) = 10-0.7
atm; P (N2) = 10-0.1 atm
H2CO3 pKa1 = 6.3; pKa2 = 10.3
[Alk] = [HCO3-] + 2 [CO3-2] + [OH-] - [H+]
[Acy] = [HCO3-] + 2 [H2CO3] - [OH-] + [H+]