Chapter 5 FE EXAM FORMATTED PROBLEMS
5-1.
The pH of a finished water from an excess lime softening process is 11.24. What volume
of 0.0200 N sulfuric acid, in millimeters, is required to neutralize 1.00 L of the finished
water? Assume the buffering capacity is zero.
Assume only OH is present, then from Eq. 5-29
pOH 14.00 11.24 2.76
OH 10 2.76 1.74 10 3 moles / L
Recognizing that n=1 for OH and that normality = molarity and that
N x ml = N x ml
Then,
(1.74 10 3 )(1,000 ml ) (0.0200 N )(ml of acid)
ml of acid 86.89 or 87 ml
5-2.
What pH is required to precipitate all but 0.20 mg/L of the iron from raw water with an
Fe3+ concentration of 2.1 mg/L? Assume the temperature is 25°C. The reaction is
Fe3+ + 3OH- ↔ Fe(OH)3 ↓ and pKs= 38.57
GMW of Fe = 55.847
Ks= 10-38.57 = 2.69 x 10-39
Using Eq. 5-21
Ks = [Fe][OH]3
1/ 3
39
OH 2.69 10 6
3.58 10
9.09 10 12 moles / L
pOH log( 9.09 10 12 ) 11.04
pH 14.00 11.04 2.96
5-3.
Estimate the pH of water that contains 0.6200mg/L of carbonic acid. Assume that
[H+] = [HCO3-] at equilibrium and that the dissociation of water may be neglected. The
water temperature is 25°C, pKa1 = 6.35 and pKa2 = 10.33. GMW of H2CO3 = 62 g/mol
Converting mg/L to moles/L
0.6200 mg / L
1.00 10 5 mole / L
62 g / mol 1000 mg / g
Using Eq. 5-33 to solve the equilibrium expression for Ka1
K a1
[ H ][ HCO3 ]
10 6.35 4.467 10 7
[ H 2 CO3 ]
Assuming that [H+]=[HCO3-] then,
[ H ]2
1.00 10 5
[ H ]2 4.467 10 12
4.467 10 7
[ H ] 2.113 10 6
pH log( 2.113 10 6 ) 5.67
5-4.
Estimate the approximate alkalinity, in mg/L as CaCO3, of water with a carbonate ion
concentration of 17.0 mg/L and a bicarbonate ion concentration of 111.0 mg/L.
Calculate the equivalent weight
CaCO3
100
50equiv
2
60
30equiv
2
61
HCO3
61equiv
1
Convert to mg/L as CaCO3 using Eq. 5 - 40
CO3
50
CO3 (17 mg / L) 28.33mg / L as CaCO3
30
50
HCO3 (111mg / L) 90.98mg / L as CaCO3
6
Calculate approximat e alkalinity remembering that the 2 multiple for CO 32- has already been accounted for
Alk 28.33 90.98 119.32 or 119 mg/L as CaCO3
0
