KEY CHEM 322 – study questions for test 1
Fill in the blank.
a) If the absorbance of a 0.020 M sample is 0.150 in a 2.0 cm cell, what is the absorbance of a
0.010 M sample of the same type in a 1.0 cm cell? _______________
A
0.150

 3.75 cm1 M1
bc 2.00 cm0.0200 M
A = abc = (3.75 cm-1M-1)(1.00 cm)(0.010 M) = 0.0375
a
b) By what factor is the signal-to-noise ratio improved if 1000 spectra are averaged _____.
improvement in S/N = N  1000  31.6
c) At a particular wavelength, a solution that is 0.010 M in A has an absorbance of 0.26 and a
solution that is 0.10 M in B has an absorbance of 0.16. What is the absorbance of a solution
that is 0.010 M in A and 0.10 M in B? ______________
e) If a spectrophotometer reads 92% T with the blank and 53% T with the sample, what is the
absorbance of the sample? ________
Iblank
 0.92
I0
Isample
 0.53
T
Isample
Iblank
I0
A   log0.576  0.240

0.53
 0.576
0.92
f) Two emission lines occur at 772.0 and 772.8 nm. Determine how many lines a grating must
have to resolve these emission lines in second order. ________
 av e
772.4

 966
 2  1
0.8
R 966
N 
 483
n
2
resolving power required is R 
R = nN
f. For a particular determination, the slope of the calibration curve was 0.153 L/mg. The
standard deviation of the noise level for a blank sample was measured to be 0.022. What is the
detection limit in concentration limits for this technique?
DL  c m 
ksbl
3(0.022)
mg

 0.431
-1
m
L
0.153 Lmg
g.
Sketch a typical single beam UV spectrometer. Label the various parts and show the light
path. Use the back of this page if you need more room
1
KEY CHEM 322 – study questions for test 1
h.
Sketch the 19F NMR spectrum for 31P19F31H2. Assume that JPF = 6 Hz. and JHF =2 Hz.
JPF
JHF JHF
JHF JHF
i.
What are the principal advantages and disadvantages of Fourier Transform Spectroscopy?
advantages – speed of data collection, better resolution, better wavelength
accuracy
disadvantages – data must be transformed to be interpreted
_______________________________________________________________________
j. What would allow you to distinguish the 1H NMR spectrum of CH3CH2OH from
CH3CH2OCH3?
Both have 3 main peaks but the chemical shifts will be the same. The ketone
carbon would be significantly more downshifted than any other carbon
CH3CH2OH has two triplets and an octet
CH3CH2OCH3 has a quartet, a triplet and a singlet
_______________________________________________________________________
k. What are the advantages of a double beam instrument as compared to a single beam
instrument?
double beam
free from drift, can use less stable sources and
detectors, solvent can be automatically subtracted
single beam
greater energy throughput, better S/N, few
components, cheaper, require stable sources and
detectors
l.
Briefly discuss Beer's Law. What is it and why does it sometimes fail?
_________________________________________________________________________
m. How many spectra must be averaged to increase the signal to noise ratio by a factor of 5.
improvemen t  N
5 N
N =25
________________________________________________________________________
n. What is the maximum resolution in nm for a 2048 element multichannel detector if the
wavelength range covered is 200 nm to 400 nm.
The resolution will be the range divided by the number of elements
resolution 
400  200nm  0.0977 nm
2048
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KEY CHEM 322 – study questions for test 1
___________________________________________________________________________
o. If a 0.010 M sample has a transmittance of 90% what is the concentration of a solution of
the same substance that has a %transmittance of 80%. Assume that Beer’s Law applies.
A   log0.9  0.0458 for the 0.010 M solution
A   log0.8  0.0969 for the solution that has %T = 80%
A = abc
A 0.0458
ab  
 4.58M1
c
.01M
for solution that has %T = 80%
c
A
0.0969

 0.0212M
ab 4.58M1
_________________________________________________________________________
p. (4) Convert 500 nm to the equivalent wavenumber (cm-1) ________

1  1  109 nm  m 
1
 2


  20000 cm
  500 nm  m  10 cm 
__________________________________________________________________________
q. (5) What is the energy difference between two levels if the number of molecules in the upper
state is one fourth as many as in the lower state. Assume the temperature is 500 K _______.
 E1 E0 
kT
n 
E
ln 1   
kT
 n0 
n 
 1
E  kT ln 1    1.38  1023 JK 1 500K ln   9.57  1021 J
 4
 n0 
n1
e
n0


___________________________________________________________________________
Select the technique or techniques from the list below that would be appropriate for each of the
following analyses. Indicate if any special sample preparation is needed. (transmission IR,
reflectance IR, reflectance near-IR, Raman, UV-vis, fluorescence, NMR)
a) determine if a water solution has a small benzene impurity
UV-visible spectroscopy
Raman spectroscopy may be useful but it may not be sensitive enough
IR is sensitive but not useful for aqueous solutions
b) determine the CO2 content of an air sample.
IR spectroscopy
c) distinguish between 1,2- and 1,3- dichlorobenzene
NMR spectroscopy
d) determine Fe3+ concentration in a water sample
UV-visible spectroscopy
_____________________________________________________________________________
Which one of the following statements is FALSE
At the detection limit, the signal-to-noise ratio is about 3
The slope of a calibration curve is the sensitivity
One of the reasons to use an internal standard is to eliminate matrix effects
The accuracy of a Beer’s law plot decreases at both high and low absorbance.
3
KEY CHEM 322 – study questions for test 1
Which of the following is NOT an advantage of Raman spectroscopy as compared to IR
spectroscopy.
preparing a sample is usually easier in Raman
the Raman detection limit is lower
biological samples are easier to study in Raman
Raman is better for detecting symmetric vibrations
A typical source for an IR experiment is
a globar
a tungsten lamp
an RF coil
a D2 lamp
The Raman shift for a CH vibration is 3000 cm-1 when the spectrum is excited with 500 nm
radiation. The Raman shift for the same vibration excited with 250 nm radiation is
3000 cm
250 cm
1500 cm
6000 cm

Which of the following techniques would be LEAST useful in determining if a compound is a
ketone
fluorescence
IR
Raman
NMR
The x axis of an IR spectrum usually has units of
wavenumbers
nm
parts per million
Which of the following techniques is LEAST used as a quantitative method
fluorescence
UV-Vis
IR
eV
NMR
the 19F NMR spectrum for 13C19F4 would look like which of the following
Beers Law is more accurate in UV-Vis than in the IR because
UV-Vis detectors are more sensitive than IR detectors
the absorption bands in the UV-Vis are typically much stronger than in the IR
the absorption bands in the UV-Vis are typically much broader than in the IR
the energy of the photon is higher in the UV-Vis
Which of the following will be TRUE for an NMR spectrum taken on a 200 MHz instrument as
compared to a 100 MHz instrument
the chemical shifts are twice as big
the spin-spin coupling constants are twice as big
the chemical shifts and the spin-spin coupling constants are the same on both
both the chemical shifts and the spin-spin coupling constants are twice as big
When a substance in a solid or solution exhibits fluorescence, the light emitted is of
a shorter wavelength than that absorbed
a longer wavelength than absorbed
a higher energy than absorbed
4
KEY CHEM 322 – study questions for test 1
the same intensity as that absorbed
The mid-IR covers the range
200 - 700 nm
400 - 4000 cm
4000 - 14000 cm
20 - 400 cm
In Raman scattering
lasers are used as sources and photomultipliers as detectors
tungsten lamps are used as sources and photomultipliers as detectors
lasers are used as sources and thermocouples as detectors
lasers are used as sources and bolometers as detectors
Which of the following is NOT a type of multichannel detector
photodiode array
charge coupled
photographic film
device
photomultiplier

In the figure shown above, which arrow corresponds to Raman emission?
C
A
F
H
In the figure shown above, which arrow corresponds to UV or visible absorption?
A
B
G
H
In the figure shown above, which arrow corresponds to IR absorption?
B
A
F
H
In the figure shown above, which arrow corresponds to fluorescence?
F
A
C
H
Which of the following techniques is LEAST used as a quantitative method
5
KEY CHEM 322 – study questions for test 1
fluorescence
UV-Vis
NMR
near-IR
Which of the following types of electronic transitions usually occurs at the lowest energy
n
 


Which substance could not be detected by infrared spectroscopy
NaCl
water
benzene
Which of the following is not a detector for visible light
MCT detector
vacuum phototube
photomultiplier
p.
ethanol
silicon photodiode
Which of the following is NOT a nucleus commonly studied in NMR
16
19
13
O
F
C
1
H
The signal observed in FT-NMR is called the
spin-spin relaxation
free induction decay
spin-lattice relaxation
Fourier transform
Molar absorptivity varies with
wavelength
cell path length
concentration
detector
Which of the following pair of spectroscopic techniques would be most useful for the
determination of the structure of an unknown organic compound.
NMR and IR
NMR and UV-Vis
IR and UV-Vis
UV-Vis and fluorescence
The term that refers to the raw signal observed in FT-NMR is
free induction decay
spin-spin splitting
spin-lattice relaxation
DEPT
n FT-NMR the sample is irradiated with pulses of RF radiation because
a short pulse can simultaneously excite all nuclei of a given type
a pulse of high power radiation is more efficient than continuous radiation
at the power levels used continuous radiation would cause saturation
the use of pulses permits greater precision in the frequency that is applied
Which group of instruments are most likely to be able to use the same type of detector.
IR, Raman and UV-Vis
Raman, fluorescence and UV-Vis
IR, fluorescence and AA
IR, fluorescence and UV-Vis
Which one of the following pairs are common IR detectors?
MCT and DTGS
PMT and MCT
DTGS and CCD
6
PMT and CCD
KEY CHEM 322 – study questions for test 1
Which part of the electromagnetic spectrum is used in NMR spectroscopy?
visible
microwave
UV
radio wave
Which one of the following parts of the IR spectrum tends to have the most information on
characteristic group frequencies
200 - 800 nm
400 - 2000 cm
4000 - 14000 cm
20 - 400 cm
Which of the following terms describes a radiationless processe for an electronic transition from
a singlet to triplet state?
intersystem
fluorescence
phosphorescence
internal
crossing
conversion
Which one of the following is a correct order in terms of increasing energy for photons
radio wave < infrared < visible < UV
radio wave < visible < UV < infrared
radio wave < UV < infrared < visible
infrared < visible < UV < radio wave
Which one of the following most affect the resolution in FTIR spectroscopy?
The type of detector
The number of scans that is averaged
The distance that the mirror moves
The slitwidth
The x axis of an IR spectrum usually has units of
cm-1
nm
parts per million
eV
Which one of the following vibrations would result in a strong peak in the IR spectrum but a
weak peak in the Raman? The arrows indicate the movement of the atoms in the vibration.
O
O
A calibration curve (A vs c) has the equation A = 0.174c + 0.021. Which expression should be
used to determine the concentration of an unknown that has an absorbance of 0.400?
0.400  0.021
0.174
0.400  0.021
0.174
0.400  0.174
0.021
0.400  0.0174
How many peaks are in the 13C NMR spectrum of the following compound?
Br
Br
.
2
3
4
7
5
KEY CHEM 322 – study questions for test 1
Which one of the following is FALSE about NMR spectroscopy?
Increasing the strength of the magnet improves resolution
Increasing the strength of the magnet improves sensitivity
chemical shift is related to the spins of nearby nuclei
the NMR peaks for solid samples tend to be very broad
8