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Chem 324: Physical Methods Of Structure Determination Question Paper
Chem 324: Physical Methods Of Structure Determination
Course:Bachelor Of Science & Bachelor Of Education (Science)
Institution: Chuka University question papers
Exam Year:2013
CHUKA
UNIVERSITY
UNIVERSITY EXAMINATIONS
THIRD YEAR EXAMINATIONS FOR THE AWARD OF DEGREE OF
BACHELOR OF SCIENCE & BACHELOR OF EDUCATION (SCIENCE)
CHEM 324: PHYSICAL METHODS OF STRUCTURE DETERMINATION
STREAMS: B.SC & BED (SCIE) Y3S2 TIME: 2 HOURS
DAY/DATE: THURSDAY 25/4/2013 2.30 PM – 4.30 PM
INSTRUCTIONS:
ANSWER QUESTION ONE AND ANY OTHE TWO QUESTIONS
USEFUL DATA:
Avogadro’s constant = 6.0×?10?^(23 ) ?mol?^(-1)
Velocity of light =3×?10?^8 ?ms?^(-1) c=12,0=16
Nuclear magneton (ß)=5.051×?10?^(-24)
Magnetic moment of the proton (µ)in
Magneton =2.7927 nuclear mangnetons
Spin quantum number (I) for proton =1/2
Boltz mann constant (K)=1.38×?10?^(-16 eng/deg)
Plank’s constant (h)=6.625×?10?^(-27 erg-sec.)
0?=237k
QUESTION ONE (30 MARKS)
1. (a) (i) With reasons explain the difference between a mole cular absorption
spectrum and an atomic absorption spectrum. [3 Marks]
(ii) Explain how particle property of electromagnetic radiation accounts for absorption of radiation in electronic transition [2 Marks]
(iii) Stray radiation which is 1.5% of the incident power,P_O, passes through the reference cell. Calculate the absorbance error which occurs for a sample with transmittance of 10% [2 Marks]
(iv) Explain why the monochromation in the ultra violet spectrophotometers is always placed after the radiant source of radiation while it is placed immediately after the sample in the infra-red spectrophotometer.
[2 Marks]
(v) Briefly explain why good detector possess short response time.
[1 Mark]
(b) (i) Describe how the following molecule may be distinguished
spectroscopically.
H_2 N ¨ ?Cl?^- H_3
(I) (II) [2 Marks]
(ii) Three organic compounds Z, Y and Z were analysed and the following
observations were made:
(a) Both X and Y are I.R active
(b) Both Y and Z are U.V active
(c) Z absorbs U.V radiation at a longer wavelength than Y.
State the conclusions that would be made from these observations about the compounds X, Y and Z. [2 Marks]
(iii) 30cm3 of a 0.03M standard solution was diluted with an equal volume of distilled water to colour match an unknown sample. Calculate the molar concentration of the sample. [1 Mark]
(c) (i) Calculate the approximate wave number and wavelength of the
fundamental absorption peak due to the stretching vibration of a carbonyl.
Group C=0 (The force constant for the typical double bond is about
1×?10?^6 dynes/cm) [3 Marks]
(ii) How many fundamental vibrational frequencies would you expect to observe in the infrared absorption spectrum of CO2. [2 Marks]
(d) (i) Calculate the relative number of protons in the higher and lower magnetic
states when a sample is placed in a 10,092 G field at 20? in NMR instrument. [3 Marks]
(ii) At what frequency would the proton Nucleus absorb in the above field
(d(i))? [2 Marks]
(e) (i) Why must a mass spectrometer utilize a sophicated high vacuum system?
[1 Mark]
(ii) Briefly discuss the value of the mass spectrum as a tool for qualitative
analysis. [2 Marks]
(iii) (a) What do you understand by resolution of a mass spectrometer?
[½ Mark]
(b) List any of the three factors which contribute to a decrease in resolution. [1 ½ marks]
QUESTION TWO (20 MARKS)
2. (a) (i) State the rules governing the appearance of first order spin – spin spectra
of NMR. [7 Marks]
(ii) For each of the following compounds, calculate the number of multiplets
for each band and their relative areas. [3 Marks]
(a) cl ?CH?_2 ?CH?_2 ?CH?_(2 ) cl (b) ?CH?_3 CH Br?CH?_3 [2 Marks]
(b) ?CH?_3 ?CH?_(2 ) O? CH?_3 [3 Marks]
(b) Calculate the ratios of the (M+1)^+ to M^+ peak heights for the following two
compounds:
(i) dinitrobenzene
C_6 H_4 N_2 O_(4 ) (M=168) [2 ½ Marks]
(ii) Olefin
C_12 H_(2 4 ) (M=168) [1 ½ Marks]
(iii) Comment on your answers in b(i) and b(ii) [1 Mark]
Element Most Abundant Isotope Abundance of the other Isotopes Relative to 100 parts of the most Abundant
Hydrogen
Carbon
Nitrogen
Oxygen 1H
12C
14N
16O 2H
13C
15N
17O 0.015
1.08
0.37
0.04
QUESTION THREE (20 MARKS)
3. (a) (i) Define the term “monochromator” [ ½ Marks]
(ii) Explain the necessity of monochromation in analysis. [1 ½ Marks]
(b) (i) With the aid of a well labeled sketches show the difference between
diffraction and dispersion types of monochromators. [5 Marks]
(ii) Assuming that the limits of the visible spectrum are approximately 380
and 700 nm, determine the angular range of the first order visible spectrum produced by a plane grating that has 900 grooves /mm with the light incident normally on the grating. [3 Marks]
(c) (i) Derive equations for the spectrophotometric determination of the
concentration of three substances present in a solution if the substance have the following specific absorptivity “a” at the following wavelengths. Express concentration in milligrams per ml. given the cell thickness is 1 cm.
Substance Wavelength
400 nm 500 nm 600 nm
A
B
C 0
2.00
0.60 0
0.05
1.80 1.00
0
0
(ii) State and explain the causes of deviation from beer-Lambert law.
[4 Marks]
QUESTION FOUR (20 MARKS)
4. (a) Describe briefly spectrophotometric elucidating the composition of complex in
solution using mole ration method. [2 Marks]
(b) Combination of solutions containing the Ligand L and the cation M2+ results in
formation of complex with an absorption maximum at 520 nm. A series of solutions is prepared in which the molar concentration of M2+ is held constant at 1.35×?10?^(-4) M while the concentration of L is varied. The absorbance data for these solutions are as follows:
Concentration of L Moles/ Litre Absorbance (?max 520 nm(1.00 cm cells)
3.00×?10?^(-5) 0.068
7.00×?10?^(-5) 0.163
1.20×?10?^(-4) 0.274
2.00×?10?^(-4) 0.419
3.00×?10?^(-4) 0.514
4.00×?10?^(-4) 0.573
5.00×?10?^(-4) 0.608
6.00×?10?^(-4) 0.626
7.00×?10?^(-4) 0.630
8.00×?10?^(-4) 0.630
(i) Evaluate the composition of the complex. [6 Marks]
(ii) Calculate a value of stability constant for the complex. [3 Marks]
(c) (i) Outline the procedure for the detection and identification of impurities by IR
spectroscopy. [3 ½ Marks]
(ii) Both benzene and cyclo hexane have C-H bonds
(a) Sketch IR spectrum expected for a mixture of these compounds.
[1 Mark]
(b) Indicate clearly the absorption band due to each compound. [1 Mark]
(c) Explain why the molecules absorb at different wave number
[2 Marks]
(iii) Determine the frequency of the fundamental absorption given that its third
overtone was observed at 3640 mc-1. [1 ½ Marks]
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