Chemistry, Third edition

REVISED QUESTIONS 397

(iii)The colour of paints may also be explained using the

colour cheese to predict which frequencies of light are

absorbed and which are reflected. Explain why when lots of

different paints are mixed, a black colour results.

20.10.The following table lists the molar absorption co-

efficient of ozone, O 3 (g) at 298 K in part of the ultraviolet region.

Wavelength/nm Molar absorption
coefficient/mol–1m^2

210 11

220 35

230 101

240 204

250 298

254 369

260 298

270 204

280 101

290 35

300 11

Plot!against wavelength. Calculate the absorbance at 254 nm

of a 10.0 cm gas cell containing pure ozone at a pressure of

100.0 Pa at 298 K. (Hint Use the ideal gas equation to convert

gas pressure to gas concentration.)

20.11.The simplified (a)mass spectrum and (b)infrared

spectrum of an unknown compound are shown below.

Additional information:

(a)The low resolution proton NMR spectrum (not shown)

contains two signals.

(b)The compound contains 37.5% C, and 12.5% H; testing

for other elements was not carried out.

Suggest a structure for the unknown compound.

20.12.Hydrocarbon concentrations in water can be mea-

sured by extracting the hydrocarbons from the water with a

suitable solvent (such as trifluorotrichloroethane (CF 3 CCl 3 ),

and subsequently measuring the absorbance of the hydro-

carbon solution in the C–H stretch region at 2860 cm–1.

(i) Why should chloroform CHCl 3 not be used as the solvent

in such measurements?

(ii)In an experiment, octane from a water sample was com-

pletely extracted using an equal volume of CF 3 CCl 3

solvent. The absorbance of octane in solution in a 20 mm

cell was found to be 0.050 at 2860 cm–1. Given that

!(C 8 H 18 ) in CF 3 CCl 3 at 2860 cm–1is 5.0 mol–1m^2 ,

calculate the concentration of octane in the original water

sample in (a) mol m–3and (b) mg dm–3.

20.13.Chromate(VI) ions (CrO 4 2–(aq)) may be completely

reduced to chromium(III) ions (Cr3+(aq)) in the presence of

thiourea catalyst and a sodium ethanoate–ethanoic acid

buffer. The reaction may be simply expressed as

CrO 4 2–(aq) + 8H+(aq) + 3e–→Cr^3 (aq) + 4H 2 O(l)

(i) By working out the oxidation numbers of chromium in

the two species, show that reduction has taken place.

(ii)At 440 nm, (CrO 4 2–(aq)) = 200.0 mol–1 m^2 and

!(Cr3+(aq)) = 1.0 mol–1m^2. In an experiment, a solution

initially containing 5.0  10 –4mol dm–3chromate(VI) ion

was completely reduced to chromium(III) ions. Assuming

a cell pathlength of 1.0 cm, what will be the initial and final

absorbance of the reaction mixture at 440 nm? (Assume

that the thiourea and buffer do not absorb at 440 nm.)

20.14.Flame atomic absorption spectroscopy (AAS) is a

specialized spectroscopic technique widely used in analytical

laboratories to find the levels of trace metals in solution. By

consulting a textbook on analytical chemistry, find out (i)

what light sources are used in AAS and (ii) the purpose of the

flame.

20.15.Identify the compound, of molecular formula

C 2 H 4 Br 2 , whose^1 H-NMR spectrum is shown below:

Absorbance

300 400 500 700

Wavelength/nm

100%

10

m/e

20 30 40

28

15

29

31

32

Transmittance

4000

3000 2000 1000

460

Wavenumber/cm–1

C 2 H 4 Br 2

5.00 0.00

TMS

δ