Practice Quiz

CLEP Chemistry

Category - Multiple Choice

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One liter of hydrogen gas is placed in a vacuum. The emission spectrum of hydrogen shows a red line of wavelength 650 nm. What was the energy of the photon that released it?
  1. 2.9 * 10-28 J
  2. 4.1 * 10-31 J
  3. 2.9 * 10-19 J
  4. 4.1 * 10-19 J
  5. 2.9 * 10-15 J
Explanation
Answer: C, 2.9 * 10-19 J is the correct answer. All incorrect answers are based on errors in units. The photon of light that shows the red line corresponds to energy released from an excited electron. Using the wavelength, we can find the energy. The solution is as follows:

E=energy (J)
H=Planck’s constant (6.626*10-34 J*s)
f=frequency (1/s)
C=speed of light (3.0 * 108 m/s)
l = wavelength (650nm = 6.5 *10-7 m)

Equations
E=hf
f=c/l *this equation applies in a vacuum
Therefore E=hc/l

E=(6.26*10-34 J*s) * (3.0 *108 m/s)/(6.5 *10-7 m) = 2.9 *10-19 J
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