Chapter_7_Review_Problems_Key

Chapter_7_Review_Problems_Key - Chapter 7 Review Problems 1...

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Chapter 7 Review Problems 1. A laser produces a light of wavelength 575 nm. How many photons of this light would have to strike a material for the material to absorb 55.1 kJ? A . 1.59 x 10 23 B. 5.31 x 10 22 C. 3.45 x 10 20 D. 6.36 x 10 23 E. 2.76 x 10 22 E photon = hc/λ h = 6.626 x 10 -34 Js c = 3 x 10 8 m/s E photon = ((6.626 x 10 -34 Js) x (3 x 10 8 m/s))/(575 nm x (1 x 10 -9 m/nm)) = 3.457 x 10 - 19 J/photon 55100J/(3.457 x 10 -19 J/photon) = 1.59 x 10 23 photons 2. A 5.25 milliwatt (1W = 1J/s) laser produces a beam of radiation with a wavelength of 632.8 nm. How many photons are produced by the laser in 60.0 seconds? A . 1.00 x 10 18 photons B. 1.00 x 10 21 photons C. 1.00 x 10 24 photons D. 1.00 xl0 27 photons E. 1.00 xl0 30 photons 1W = 1J/s 0.00525W = 0.00525J/s = 0.315J in 60.0 seconds. E photon = hυ = hC/λ = (6.626 x 10 -34 Js x 3 x 10 8 ms-1)/(632.8 nm x (1 x 10 -9 m/nm)) E photon = 0.0314 x 10 -17 J 0.315J/(0.0314 x 10 -17 ) = 1.00 x 10 18 photons 3. What is the wavelength of light emitted when an electron in the third energy level goes down to the ground state in the hydrogen atom? A . 103 nm B. 205 nm C. 421 nm D. 630 nm E. 821 nm
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∆E = -B((1/(n f ) 2 ) – (1/(n i ) 2 )) ∆E = -(2.179 x 10 -18 ) x ((1/(1) 2 ) – (1/(3) 2 )) = -1.937 x 10 -18 J λ = hc/E λ = ((6.626 x 10 -34 Js) x (3 x 10 8 m/s))/(1.937 x 10 -18 J) = 1.026 x 10 -7 m 1.026 x 10 -7 m x (1x10 9 nm/m) = 102.6 nm
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