TestsFall2003 - ENSC 3313, Fall 2003 TEST 1 Name_ Answer...

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ENSC 3313, Fall 2003 TEST 1 Name ________________________________ Answer all 10 questions, 2 points each . 1. The element Cf 254 is sold by the Atomic Energy Commission for $10/microgram (one millionth of a gram) and has a half-life of 55 days. Please consider a one-milligram sample. If the sample is prepared and shipped via overnight delivery, what is the value of the material that decays during the 24 hours of shipping? λ = t o e A A λ = t A A ln o () day 35 . 79 55 5 . 0 ln = λ λ = g 5 . 987 e g 1000 A 35 . 79 1 µ = µ = 125 $ g 10 $ g 5 . 987 1000 = µ × µ 2. CdS can be used as a light sensor due to its absorption of impingent photons and the ensuing creation of electron-hole pairs. If the sensor "sees" light with wavelengths < 500 nm (i.e., green light), estimate the band gap energy of this material. Note: the speed of light is c = 3 × 10 8 m/s and Plank's constant is h = 4.13 × 10 -15 eV s. The energy of a photon is given by: eV 48 . 2 10 500 10 3 10 13 . 4 c h h E 9 8 15 = × × × = λ = ν = Since only photons with enough energy to promote an electron from the valence to conduction band are absorbed, this is approximately equal to the CdS band gap energy.
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3. If we had 2 SiO 4 tetrahedra sharing an edge, as sketched, what would be the chemical formula and charge on the unit (Si x O y ) n– . (--- what are the values of x, y and n?) There would be an Si 4+ at the center of each tetrahedra ( ). Two corners are shared – there are 6 oxygen (O) total; two share with 2 silicon, hence 4 of oxygen lack one bond, hence we have 4 . Hence (Si 2 O 6 ) 4– . 4. Identify 6 planes of symmetry in a regular tetrahedron. Draw a regular tetrahedron in a cube, thence draw a cube face diagonal, of which there are 6. The cube face diagonals correspond to a tetrahedron edge. 5. Show by calculation that NaCl is a 6-fold coordination compound. The ionic radii are Na + = 0.102 nm, Cl = 0.181 nm. Sketched is the top face of the 6-fold coordination cell, which is cubic. 2r + 2R = a = 2(0.102 + 0.181) = 0.566 nm But the large ions must not touch along the face diagonal. Hence, ( ) ( ) , 800 . 0 724 . 0 , 566 . 0 2 181 . 0 4 , a 2 R 4 < < <
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6. Identify the plane and direction shown. Origin at 0 Extrapolate ‘x’ axis Intercepts are 1 2 1 1 Take reciprocals; plane = (121) Direction = 1 2 1 0 Clear fraction = [ ] 2 01 Note direction lies in plane and 0 2 1 1 2 0 2 = + + , . 7. Calculate the packing factor of CsCl. The ionic radii are Cs = 0.169 nm and Cl = 0.181 nm. Recollect that CsCl does not have the same structure as NaCl (or LiF)! cell of Volume ions of Volume PF = One kind of ion is at the cell corner, the other at the cell center. There is, on average, one of each type per cell. They contact along the cube diagonal, so a 3 r 2 R 2 = + () 68 . 0 3 181 . 0 2 169 . 0 2 181 . 0 169 . 0 3 4 . 1 PF 3 3 3 = + + π =
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8. The maximum solubility of tin in a Cu-Sn bronze is 15.8 w/o at 580 ° C. To what a/o Sn does this correspond? The amu are 63.54 and 118.69 respectively.
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This note was uploaded on 01/10/2012 for the course ENSC 3313 taught by Professor Staff during the Fall '11 term at Oklahoma State.

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TestsFall2003 - ENSC 3313, Fall 2003 TEST 1 Name_ Answer...

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