Assignment 2

CHEM250, Analytical Chemistry ________________________________________________________________

Assignment 2 Due date: Friday 28 September

Weight: 12.5% (80 marks)

Chapters: 1-4, 6, 9 – 12, 15, 30 and 31

Each question is worth 10 marks. Show ALL working.

1. (a) Given that the solubility of O2 at 0oC is 14.7 ppm, calculate the value of KH at this

temperature.

(b) A 25 mL sample of river water was titrated with 0.0010 M Na2Cr2O7 and required 8.3 mL to

reach the endpoint. What is the chemical oxygen demand, in milligrams of O2 per liter, of the

sample?

(c) The COD of a water sample is found to be 30 mg of O2 per liter. What volume of 0.0020 M

Na2Cr2O7 will be required to titrate a 50-mL sample of the water?

2. (a) Describe the processes by which the dual bed “three-way catalyst” works to transform air

pollutants released by an automobile engine.

(b) Deduce the balanced equation in which ammonia reacts with nitrogen dioxide to produce

molecular nitrogen and water. Calculate the mass of ammonia that is required to react with

1000 L of air containing 10 ppm of NO2•. Assume the air is at 27°C and 1.0 atm.

(c) In a related technology reduced nitrogen in the form of the compound urea, CO(NH2)2, is

injected directly into the combustion flame to combine there, rather than later in the presence

of catalyst, with NO• to produce N2. Deduce a balanced equation that converts urea and nitric

oxide and oxygen into N2, CO2, and water.

3. (a) The H° for the decomposition of ozone into O2 and atomic oxygen is + 105 kJ mol-1

:

O3 O2 + O

What is the longest wavelength of light that could dissociate ozone in this manner? By

reference to Figure 1-2 (p. 31 Baird & Cann), decide the region of sunlight (UV, visible, or

infrared) in which this wavelength falls.

(b) By reference to the information in part (a), calculate the longest wavelength of light that

decomposes ozone to O* and to O2*, given that the excited states of atomic and molecular

oxygen lie 190 and 95 kJ mol-1

respectively above their ground states.

(c) Using the enthalpy of formation data given below, calculate the maximum wavelength, which

can dissociate NO2 to NO and atomic oxygen. Recalculate the wavelength if the reaction is to

result in the complete dissociation into free atoms (i.e., N + 2O). [Hint: Recall that for any

reaction the enthalpy change equals the sum of the enthalpies of formation, Hf°, of the

products minus those of the reactants.]

Hf° values (kJ mol-1

): NO2: +33.2; NO: +90.2; N: +472.7; O: +249.2

Assignment 2

________________________________________________________________ CHEM250, Analytical Chemistry

4. (a) For aqueous solutions

(i) convert 0.04 g/L to the ppm and ppb scales, and

(ii) convert 3ppb to g/L

(b)

The forms shown above for DDT and DDE have both the ring chlorines in the para position,

and sometimes labeled p,p’-DDT and p,p’-DDE, where the prefixes refer to the chlorine

positions in the first and second rings respectively. Draw and deduce the appropriate labels

for all the other unique isomers of both DDT and DDE. Note that the two rings are

equivalent, so that the example o,m’-DDT is the same compound as is m,o’-DDT.

(c) What were the main uses of DDT? Explain why it is no longer used in many developed

countries.

5. (a) Explain why mercury vapour and methylmercury compounds are much more toxic than other

forms of the element.

(b) The half-life of methyl mercury in the human body is estimated to be close to 100 days.

Suppose a person weighing 65 kg eats 0.5 kg of fish containing 0.5 μg g–1

mercury three times

a week over an extended time. What is the steady-state average concentration of mercury in

this person’s body?

(c) A quantity of a mercury-chlorine compound is included in a shipment of waste to a toxic

waste disposal dump. Before it can be disposed of properly, the owners of the dump need to

know whether the compound is HgCl2, or Hg2Cl2, or some other compound. They send a

sample of it for analysis, and find that it contains 26.1% chlorine by mass. What is the formula

of the compound.

6. (a) The stepwise formation constants for the complexes Pb(OH)+ (aq) and Pb(OH)2 (aq) from

Pb2+

(aq) are 2.0 106 and 4.0 10

4, respectively. The reactions can be written in simple form

as

Pb2+

(aq) + OH– (aq) PbOH

+ (aq)

and

PbOH+ (aq) + OH

– (aq) Pb(OH)2 (aq)

Calculate the pKa1 and pKa2 values for the deprotonation of the aquo complex of lead (II)

(Pb(H2O)22+

), and determine the fractional concentration of the two most important species at

pH 7.0. [Hint: PbOH+ and Pb(H2O)(OH)

+ are the same species]

(b) Discuss the toxicity of lead, especially with respect to its neurological effects. Which

subgroups of the population are at particular risk from lead?

C ClCl

CCl Cl

DDE

C ClCl

H

CCl Cl

Cl

DDT

Assignment 2

CHEM250, Analytical Chemistry ________________________________________________________________

7. (a) Explain why, atom for atom, stratospheric bromine destroys more ozone than does chlorine.

(b) Given that diatomic chlorine gas is the stablest form of the element, and that the Hf° value

for atomic chlorine is +121.7 kJ mol-1

, calculate the maximum wavelength of light that can

dissociate Cl2. Does such a wavelength correspond to light in the visible or the UV-A or the

UV-B region?

(c) Why is the mechanism involving dichloroperoxide of negligible importance in the destruction

of ozone, compared to that which proceeds by ClO• + O, in the upper levels of the

stratosphere.

8. (a) Define the term aerosol, and differentiate between “coarse” and “fine” particulates. What are

the usual origins of these two types of atmospheric particles?

(b) What would be the correct PM symbol for an index that included only ultrafine particles?

What would be the PM symbol for the TSP index? Numerically, would the value for the

ultrafine component of a given air mass be larger or smaller than its TSP?

(c) List the four reasons why coarse particles usually are of less danger to human health than are

fine particles.