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Hw2_New.wpd
08/26/10
CHM 2045
SECOND PROBLEM SET
INSTRUCTOR: DON SWIETER
OFFICE: K223
PHONE: 3955303
QUESTIONS ON THE PROPERTIES OF LIGHT
1. What are the wavelengths, in meters, associated with radiation of the following frequencies?
a) 5.09 x 10 s
b) 8.6 x 10 s
c) 2.0 x 10 s
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2. Give the frequency and energy (per photon) associated with radiation of the following wavelengths.
a) 1.8 cm
b) 480 Å (1
Å
= 1 × 10
m)
c) 305 nm
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3. Give the frequency and wavelength (in meters) of radiation with the following photon energies.
a) 6.95 x 10
J
b) 5.96 x 10
J
c) 2.00 × 10
J
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4. Restate each of the following in the new units indicated and tell in which region of the spectrum it lies:
See handout at the end of this problem set for spectral regions.
Example: 5.00 10 s to J.
First, J is an energy unit and s is a unit of frequency,
í
.
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Therefore, since E = h•
í
, then, (6.6262 x 10
J•s) × (5.00 x 10 s ) = 3.31 x 10
J.
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a) 150 pm to cm
b) 4000 cm to J
c)
1.50 x 10 s to nm
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d) 10 cm to J
e) 1.0 x 10
J to s
f)
1.20 × 10 s to cm
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5. If each atom in one mole of atoms emits a photon of wavelength 650 nm, how much energy is lost?
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Express the answer in J/mol.
As a reference point, burning gaseous CH to form liquid water and gaseous
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CO produces 890 kJ/mol of heat.
6. Compare the energy of an xray photon (
ë
= 0.15 nm) with that of a microwave photon (
ë
= 1.0 mm).
7. If the human eye receives a 1.59 x 10
J signal from photons whose wavelength is 500 nm, how
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many photons have hit the eye?
8. Chlorophyll absorbs blue light,
ë
= 460 nm, and emits red light,
ë
= 660 nm. Calculate the net energy
change in chlorophyll when a single photon of 460 nm is absorbed and one of 660 nm is emitted.
9. The lowestfrequency light that will produce the photoelectric effect on a material is called the
threshold frequency.
a. If the threshold frequency for Pt is 1.3 x 10 s , what is the energy of a quantum of this radiation?
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b. Will platinum display the photoelectric effect when exposed to ultraviolet light (
ë

100 nm)?
infrared light (
ë

1.00 x 10 cm)?
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c. What would happen in both cases if the original intensities of the UV and IR light were doubled?
d. What would happen if the wavelength of UV light used was decreased.
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10. Cesium metal requires radiation with a minimum threshold frequency of
4.60 x 10 s before it can
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emit an electron from its surface via the photoelectric effect.
If cesium is irradiated with light whose
wavelength is 540 nm, what is the kinetic energy of the emitted electron?
11. Indicate whether energy is emitted or absorbed when the following electron transitions occur in
hydrogen:
a. n = 2 to n = 1
b. n = 2 to n = 4
c. ionization of an e in the n = 2 state.
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12. a) using the Bohr equation, calculate the energy of the photon emitted for each of the following
electronic transitions;
i) n = 4 to n = 3
ii) n = 3 to n = 2
iii) n = 2 to n = 1
Take note of the magnitude of the energy differences between adjacent levels as n increases.
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