978-1118741498 Chapter 3 Part 1

3.1 How many grams of NaCl would you need to add to a 1 L water sample (pH = 7) so the

ionic strength equaled 0.1M?

Solution:

3.2 You are studying the feasibility of using a reverse osmosis membrane system to desalinate

seawater (TDS = 35,000 mg/L) and inland brackish groundwater (TDS typically ranges from

1,000 to 10,000 mg/L). a) Estimate the ionic strength of the seawater and brackish water. b) A

conductivity meter provides a reading of 7,800 µmho/cm when placed in one of these water

samples. Which water source is the sample from?

Solution:

a) Use equation 3.2 to calculate the ionic strength

b) Use equation 3.3 to calculate specific conductance

3.3 Calculate the ionic strength and individual activity coefficients for a 1 L solution in which

0.02 moles of Mg(OH)2, 0.01 moles of FeCl3, and 0.01 moles of HCl are dissolved.

After the two components are placed in water, they will completely dissociate to form: 0.02 M

Mg2+, 0.01 M Fe2+, 0.02 OH–, 0.01 M H+, 0.04 M Cl– (0.03 from FeCl3 and 0.01 from HCl)

Solution:

Equation 3.1 is used to calculate iconic strength

3.4 Hydrogen sulfide is an odor-causing chemical found at many wastewater collection and

treatment facilities. The following expression describes hydrogen sulfide gas reacting with

aqueous phase hydrogen sulfide (a diprotic acid).

H2S (gas) = H2S (aqueous)

Use your understanding of chemical equilibrium and thermodynamics to determine the Henry’s

constant (moles/L-atm) for this reaction at 25oC. The change in free energy of formation at

standard conditions (units of kcal/mole) are as follows: H2S (gas) = -7.892, H2S (aqueous) = -6.54,

HS– (aqueous) = +3.01, SO42- = -177.34.

Solution:

𝐾𝐻= 𝑒−∆𝐺°/𝑅𝑇 =𝑒−1.35

3.5 The reaction of divalent manganese with oxygen in aqueous solution is given as follows:

Mn2+ + ½ O2 (aqueous) + H2O = MnO2 (solid) + 2H+

The equilibrium constant (K) for this reaction is 23.7. It has been found that a lake water sample

that contains no oxygen at 25oC, pH = 8.5, originally contained 0.6 mg/L of Mn2+. The sample

was aerated (atmospheric conditions so the dissolved oxygen concentration s 9.2 mg/L) and after

10 days of contact with atmospheric oxygen, the Mn2+ concentration was 0.4 mg/L. The

molecular weight of Mn is 55, O is 16, and H is 1. The change in free energy of formation at

standard conditions (units of kcal/mole) are as follows: Mn2+ = -54.4, O2 (aqueous) = +3.93, H2O = –

56.69, MnO2 (solid) = -111.1, H+ =0. a) Assuming that the pH remains constant during aeration,

will the precipitate continue to form after the measurement on the tenth day? Assume ideal

conditions. b) What should the Mn2+ concentration be (in moles/L) at equilibrium, assuming that

pH and presence of dissolved oxygen are the same as in part “A”? Assume ideal conditions. c)

What should the Mn2+ concentration (in moles/L) be at equilibrium if 2 x 10-3 moles/liter of

NaCl are added to the solution and the pH is adjusted to 2? (problem based on Snoeyink and

Jenkins, 1980).

Solution:

3.6 Phosphate ion reacts in water to form monohydrogen phosphate according to the

following reaction:

PO43- + H2O = HPO42- + OH–

The equilibrium constant for this reaction is 10–1.97. a) Given that this is a dilute system (so you

can assume ideal conditions), temperature is 298oK and the total combined

phosphate/monohydrogen phosphate is 10-4 M, what percentage of the total concentration is in

the phosphate ion form at pH= 11? b) Will the reaction proceed as written at pH=9 when [PO43-

]=10–6.8 and [HPO42-]=10-4 M? And if not, which direction will the reaction proceed?

Solution:

3.7 The chemical 1,4-dichlorobenzene (1,4-DCB) is sometimes used as a disinfectant in

public lavatories. At 20°C (68°F) the vapor pressure is 5.3 × 10-4 atm. (a) What would be the

concentration in the air in units of g/m3? The molecular weight of l,4-DCB is 147 g/mole. (b) An

alternative disinfectant is 1-bromo-4-chlorobenzene (l,4-CB). The boiling point of l,4-CB is

196°C, whereas the boiling point of l,4-DCB is 180°C. Which compound would cause the

highest concentrations in the air in lavatories? (Explain your answer.)

Solution:

3.8 The boiling temperatures of chloroform (an anesthetic), carbon tetrachloride (commonly

used in the past for dry cleaning), and tetrachloroethylene (previously used as a degreasing

agent) are 61.7°C, 76.5°C, and 121°C. The vapor pressure of a chemical is directly proportional

to the inverse of the chemical’s boiling point. If a large quantity of these compounds were spilled

in the environment, which compound would you predict to have higher concentrations in the air

above the site? (Explain your answer.)

Solution:

3.9 What would be the saturation concentration (mole/L) of oxygen (O2) in a river in winter

when the air temperature is 0°C if the Henry’s law constant at this temperature is 2.28 × 10-3

mole/L–atm? What would the answer be in units of mg/L?

Solution:

3.10 The log Henry constant (units of L-atm/mole and measured at 25°C) for trichloroethylene

(TCE) is 1.03; for tetrachloroethylene (PCE) it is 1.44; for 1,2-dimethylbenzene it is 0.71; and

for parathion it is -3.42. (a) What are the dimensionless Henry’s law constants for each of these

chemicals? (b) Rank the chemicals in order of ease of stripping from water to air.

Solution:

0.4. A sealed glass vial is prepared that has an air volume of 4 mL overlying an aqueous volume

of 36 mL. TCE is added to the aqueous phase so that initially it has an aqueous phase

concentration of 100 ppb. After the system equilibrates, what will be the concentration (in units

of µg/L) of TCE in the aqueous phase?

Solution:

3.12 The Henry’s law constant for H2S is 0.1 moles/L–atm and

H2S(aq) HS– + H+, Ka = 10-7

If you bubble pure H2S gas into a beaker of water, what is the concentration of HS– at a pH of 5

in (a) moles/L; (b) mg/L; (c) ppmm?

Solution:

3.13 Determine the equilibrium pH of aqueous solutions of the following strong acids or

bases: (a) 15 mg/L of HSO4–; (b) 10 mM NaOH; (c) 2,500 µg/L of HNO3.

Solution:

3.14 What would be the pH if 10-2 moles of hydrofluoric acid (HF) were added to 1 L pure water? The

pKa of HF is 3.2.

Solution:

3.15 When Cl2 gas is added to water during the disinfection of drinking water, it hydrolyzes

with the water to form HOCl. The disinfection power of the acid HOCl is 88 times better than its

conjugate base, OCl–. The pKa for HOCl is 7.5. (a) What % of the total disinfection power (i.e.,

HOCl + OCl–) exists in the acid form at a pH = 6? (b) At pH = 7?

Solution:

3.16 A 1-liter aqueous solution is prepared at 25oC with 10-4 moles of hydrocyanic acid (HCN)

and 10-3 moles of disodium carbonate (Na2CO3) and reaches equilibrium. a) List the eight

unknown chemical species here (water is not unknown). b) List (do not solve) all four

equilibrium expressions that describe this system making sure to include the value for the

equilibrium constants.

Solution: