ps 2 solution - Chemistry420 Problemset#2(80pointstotal)

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Chemistry 420 Problem set #2 (80 points total) Due at the beginning of class October 1 No late assignments will be accepted. Assignments are due at beginning of class—starts at 12 Noon! For full credit assignments must be in box when the lecture begins. Any assignment turned in between noon and 12:05 PM will automatically be deducted 5 points. Assignments will NOT be accepted for credit after 12:05 PM. Score = 0 No name, no grade. Show all of your work for each answer. Partial credit will be given and NO credit will be awarded if no work is shown. Be careful to use correct significant figures. 1. (10 points, Skoog 8 8) The Doppler effect is one of the sources of the line broadening in atomic absorption spectroscopy. Atoms moving toward the light source encounter higher frequency radiation than atoms moving away from the source. The difference in wavelength Dl experienced by an atom moving at speed v (compared to one at rest) is v c λ Δ = , where c is the velocity of light. Estimate the line width (in nanometers) of the lithium line at 670.776 nm when the absorbing atoms are at a temperature of (a) 2100 K and (b) 3150K. The average speed of an atom is given by 8 kT v m π = , where k is Boltzmann’s constant, T is the absolute temperature, and m is the mass. (2 points for correctly determining mass of lithium atom, 3 points for correctly setting up velocity calculation, 3 points for part (a) answer, and 2 points for part (b) answer; 1 point for egregious significant figure mistakes) () 23 -26 23 2 23 2 3 -26 8 1m o l e g Li atom 6.941 1.153 10 g/Li atom = 1.153 10 kg/Li atom mole Li atoms 6.022 10 kg m 8 1.38 10 2100 K s K (a) 2.53 10 m/s 1.153 10 kg/Li atom , so = kT v m m v vv c = ⎛⎞ =⋅ = ⎜⎟ ⎝⎠ ⋅⋅ == Δ⋅ ( ) 3- 9 12 -3 8 2 23 2 3 -26 9 670.776 10 m = 5.66 10 m = 5.66 10 nm 2.99792 10 m/s kg m 81 . 3 0 3 1 5 0 K (b) 3.10 10 m/s 1.153 10 kg/Li atom , so = 670.776 10 = c v cc λλ Δ= Δ 12 -3 8 m 6.94 10 m = 6.94 10 nm
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2. (3 points, Skoog 9 9) Why is atomic emission more sensitive to flame instability than atomic absorption or atomic fluorescence? The population of excited atoms from which emission arises is very sensitive to the flame temperature and other conditions. The population of ground state atoms, from which absorption and fluorescence originate, is not as sensitive to these conditions since it is a much larger fraction of the total population.
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This note was uploaded on 04/08/2010 for the course CHEM 420 taught by Professor Bailey during the Spring '08 term at University of Illinois at Urbana–Champaign.

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ps 2 solution - Chemistry420 Problemset#2(80pointstotal)

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