CEE 370

Fall 2010

Homework #2

1. How many grams of anhydrous Sodium Sulfate (Na₂SO₄) must be added to a liter of water so that the ionic strength equals 0.01M?

for a solution of sodium sulfate at a molar concentration of “C”, we have “C” moles of sulfate (a divalent ion) and “2C” moles of sodium (a monovalent ion). Therefore,

since the GFW of Na₂SO₄ is (2x23)+32+(4x16)=142

2. The pesticide, parathion, undergoes photolytic degradation (destruction by absorption of energy from the sun’s rays) in natural waters. This is a first order process with a rate constant of 8x10^-7s^-1.

a. What is the half-life for parathion based on this degradation process?

b. How long will it take for 90% of the parathion to degrade?

c. How long will it take for 99% of the parathion to degrade?

3. Hypochlorous acid (HOCl) reacts with phenol (C₆H₅OH) to produce a wide range of products including highly offensive chlorophenols. The rate of this reaction is second order, overall (first order in each reactant). The rate constant is 2x10³ M^-1s^-1. In this problem you have 1.2 ppb of phenol in water, and to this you add 0.1 mM hypochlorous acid.

a. How long will it take to destroy 50% of the phenol?

First, calculate molar phenol concentration.

Although this is a second order kinetic problem, one of the reactants (hypochlorous acid) is present at a very large molar excess (e.g., 0.1 mM vs 0.0000128mM). Therefore, this reactant will not become depleted as the reaction proceeds, and the overall reaction takes on the form of pseudo-first order kinetics.

So the following equation will hold:

Where:

and now we can simply used the re-arranged form of the first order kinetic equation as developed in question #2 above.

b. How long will it take to destroy 99.99% of the phenol?

And using the same equation again:

4. Calculate the ThOD of the following wastewaters

a. 45 mg/L of methanol

The balanced stoichiometric equation is:

The ThOD is then:

b. 10 mg/L of glucose

The balanced stoichiometric equation is:

The ThOD is then:

c. 45 mg/L of methanol with 10 mg/L glucose

The ThOD of the two components are calculated in parts a and b. The answer is the sum of the two, or:

d. 10^-4 M acetic acid

First, the mass-based concentration is:

The balanced stoichiometric equation is:

The ThOD is then:

e. 5 mg/L of trichloroethane

The balanced stoichiometric equation is:

The ThOD is then:

5. Nitrogen occurs at a variety of oxidation states in water environments. Balance the following oxidation-reduction reaction which is important in wastewater treatment:

NH₃ + O₂ → NO₃^- + H₂O

Show all steps.

Oxidation:

Reduction:

Multiply the reduction reaction by 2x and adding the two we get: