AP Chemistry : Spectroscopy and Electromagnetic Spectrum

Study concepts, example questions & explanations for AP Chemistry

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Example Questions

Example Question #111 : Solutions And States Of Matter

What is the energy of a photon that has a wavelength of 540 nm?

Possible Answers:

\(\displaystyle 7.36 x 10^{-17} J\)

\(\displaystyle 3.68 x 10^{-17} J\)

None of the above

\(\displaystyle 3.68 x 10^{-19} J\)

\(\displaystyle 3.68 x 10^{-28} J\)

Correct answer:

\(\displaystyle 3.68 x 10^{-19} J\)

Explanation:

\(\displaystyle E= \frac{hc}{\lambda}\) ; \(\displaystyle E = \frac{(6.626 \, x \, 10^{-34} J\cdot s)(3.00\, x\, 10^{-8} m/s)}{5.40 \, x \, 10^{-7} m/s}\);  \(\displaystyle E = 3.68 \, x \, 10^{-19}\, J\)

Example Question #2 : Spectroscopy And Electromagnetic Spectrum

What transition would emit the longest wavelength of light?

Possible Answers:

n = 4 to n = 1

n = 4 to n = 3

n = 3 to n = 1

Not enough information given

n = 2 to n = 1

Correct answer:

n = 4 to n = 3

Explanation:

Wavelength is inversely proportional to energy.  Since the transition from n=4 to n=3 is the lowest energy transition, it would have the longest wavelength.

Example Question #112 : Solutions And States Of Matter

What photon wavelength can promote a transition from the n = 1 (ground state) to the n = 3 (excited state)?

Possible Answers:

102 nm

204 nm

1 m

408 nm

306 nm

Correct answer:

102 nm

Explanation:

\(\displaystyle \Delta E = (-2.18 \, x \, 10^{-18} \, J)(\frac{1}{3^2} -\frac{1}{1^2}) = 1.94 \, x\, 10^{-18} \, J\)

 

\(\displaystyle E =\frac{hc}{\lambda }\)\(\displaystyle \lambda =\frac{hc}{E} = \frac{(6.626 \, x \, 10^{-34}\, J \cdot s)(3.00\, x\, 10^8 m/s)}{1.94 \, x \, 10^{-18} \, J} = 1.02 \, x\, 10^{-7} m = 102 nm\)

Example Question #4 : Spectroscopy And Electromagnetic Spectrum

A 3.0 eV light is shined on Gold ( Փ = 5.1 eV), silver (Փ =4.26 eV), Cesium (Փ = 2.1 eV), and Platinum (Փ = 6.35).  If Փ is the work function of the metal, what metal would have electrons ejected?

Possible Answers:

Cs

Au

Pt

Au, Ag, and Pt

Ag

Correct answer:

Cs

Explanation:

The work function is the energy required to remove an electron from a metal.  Only Cs metal has a work function less than the energy of the light.

Example Question #5 : Spectroscopy And Electromagnetic Spectrum

A student has a Blue #1 dye with a molar absorptivity of 1.30 x 105 M-1 cm-1 at 631 nm.  If the student runs this sample through a spectrophotometer and finds it has an absorption of 0.500 what is the dye’s molarity?  Assume a path length of 1.0 cm.

Possible Answers:

\(\displaystyle 1.9 \, x \, 10^{-5} M\)

\(\displaystyle 11.5 \, x \, 10^{-5} M\)

\(\displaystyle 3.9 \, x\, 10^{-6} M\)

\(\displaystyle 1.9 \, x \, 10^{-6} M\)

\(\displaystyle 3.9 \, x \, 10^{-5} M\)

Correct answer:

\(\displaystyle 3.9 \, x\, 10^{-6} M\)

Explanation:

\(\displaystyle A = \varepsilon bc\)\(\displaystyle c =\frac{ A}{\varepsilon b}= \frac{0.500}{(1.30\, x \, 10^5 M^{-1} cm^{-1})(1.00 cm)} = 3.9 \, x \, 10^{-6} M\)

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