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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? _______________
1 1
75 cm M
00 cm 0. 0200 M
bc
A
a
A = abc = (3.75 cm
M
)(1.00 cm)(0.010 M) = 0.
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? ________
I
I
0
blank
I
I
0
sample
I
I
T
blank
sample
A log 0. 576 0. 240
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. ________
resolving power required is
R
2 1
ave
R = nN
n
R
N
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?
L
m g
0. 153 Lmg
m
ks
DL c
bl
m
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
h. Sketch the
19
F NMR spectrum for
31
P
19
F
3
1
H
2
. Assume that J PF
= 6 Hz. and J HF
=2 Hz.
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
1
H NMR spectrum of CH 3 CH 2 OH from
CH
3
CH
2
OCH
3
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
CH
3
CH
2
OH has two triplets and an octet
CH
3
CH
2
OCH
3
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.
improvement N
5 N
N =
________________________________________________________________________
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
0. 0977 nm
400 200 nm
resolution
___________________________________________________________________________
J
P F
J
H F
J
H F
J
H F
J
H F
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 tungsten lamp a globar an RF coil a D 2 lamp
The Raman shift for a CH vibration is 3000 cm
when the spectrum is excited with 500 nm
radiation. The Raman shift for the same vibration excited with 250 nm radiation is
250 cm
1500 cm
3000 cm
6000 cm
Which of the following techniques would be LEAST useful in determining if a compound is a
ketone
IR Raman fluorescence NMR
The x axis of an IR spectrum usually has units of
wavenumbers nm parts per million eV
Which of the following techniques is LEAST used as a quantitative method
fluorescence UV-Vis IR NMR
the
19
F NMR spectrum for
13
C
19
F
4
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
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
device
photographic film photomultiplier
In the figure shown above, which arrow corresponds to Raman emission?
A C 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?
A B F H
In the figure shown above, which arrow corresponds to fluorescence?
A C F H
Which of the following techniques is LEAST used as a quantitative method
fluorescence UV-Vis near-IR NMR
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
crossing
fluorescence phosphorescence internal
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
-
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.
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?
How many peaks are in the
13
C NMR spectrum of the following compound?
Br Br
O O
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
