Ch 8 Online Examples Answers Pt1

# Ch 8 Online Examples Answers Pt1 - HODSON CHE 1301 Petrucci...

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HODSON CHE 1301 Petrucci Ch8 Example Problems The following problems are taken from old exams . ..... 1. How many photons of light with frequency 5.50 × 10 15 Hz are required to provide 1 kJ of energy? a)   2.74 × 10 20 photons b)   4.56 × 10 –4 photons c) 1.65 × 10 44 photons d) 3.64 × 10 –18 photons e) 3.64 × 10 –16 photons Answer: A Explanation: Here we have a ‘remember the equations and plug in the numbers’ question. The energy of a photon is given by Plank’s constant multiplied by the frequency of the photon (E = h v ). The frequency is given in Hz, which is just s –1 . Note that h is in terms of Joules, not kilojoules, so we’ll have to convert. Once we have the energy of 1 photon, we can use this equivalence to convert 1 kJ of energy to a number of photons. E = h v = (6.626 × 10 –34 Js) × 5.50 × 10 15 s –1 = 3.64 × 10 –18 J, therefore 3.64 × 10 –21 kJ = energy of 1 photon 1 kJ × (1 photon / 3.64 × 10 –21 kJ) = 2.74 × 10 20 photons 2. What is the energy in joules of the 656 nm spectral line of hydrogen. a)   1.45 × 10 –48 J b) 1.30 × 10 –22 J c) 3.03 × 10 –19 J d) 3.03 × 10 –28 J e) 4.35 × 10 –31 J Answer: C Explanation: We have to relate energy to wavelength of a line in the hydrogen emission spectrum. We do this by combining the equations for the energy of a photon (E = h v ) and the characterization of a wave (c = λ v ). From the latter, v = c / λ , and so the energy associated with the line can be calculated using: E = hc / λ (the above equations and this one are obviously good to remember) E = hc / λ = [(6.626 × 10 –34 Js) × 3.00 × 10 8 m/s] / 656 × 10 –9 m (Note that we have to convert nm to m, because Planks equation is in terms of Js, which is essentially kg m 2 s –1 ) E = 3.03 × 10 –19 J 3. Calculate the wavelength, in meters, of light absorbed by an electron in an atom of

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## This note was uploaded on 12/19/2009 for the course CHE 1301 taught by Professor Klausmeyer during the Spring '08 term at Baylor.

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Ch 8 Online Examples Answers Pt1 - HODSON CHE 1301 Petrucci...

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