exam1spring2004 - ENSC 3313, Spring 2004 TEST 1 - KEY Name_...

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ENSC 3313, Spring 2004 TEST 1 - KEY Name_________________________ Answer all 10 questions, show all work, 2points each 1. The naturally occurring isotope C 14 decays exponentially with time. If 25% of the original amount of C 14 has decayed in 2378 years, calculate the time required for an additional 5% (i.e., 30% total) to decay. Answer Right away we can write: τ / 1 75 . 0 t o e m m = = and / 2 70 . 0 t o e m m = = Next, we can solve each for time ) 75 . 0 ln( 1 = t and ) 75 . 0 ln( 2 = t and combine to find t 2 in terms of t 1 1 2 2 1 241 . 1 806 . 0 ) 7 . 0 ln( ) 75 . 0 ln( t t t t = = = I'm looking for the time interval to go from 25% to 30% decayed so years 573 241 . 0 ) ( 1 1 2 = t t t 2. Assuming that the (110) plane in a copper FCC crystal is exposed as an external surface, how large of an area would be needed to expose 0.1 moles of atoms on this surface? Note: atomic radius of 0.128 nm and N A = 6.023 × 10 23 . Answer The number of atoms per unit cell on the (110) plane is 2 (i.e., count ¼ of the 4 corner atoms and ½ of the 2 face atoms). Now we calculate the area of this plane. 2 ) 110 ( 2 8 ) 4 )( 2 2 ( 2 R R R a a A = = × = 2 2 8 . 10 ) 128 . 0 ( 2 8 2 # nm atoms area atoms PD = = = Thus the area of the exposed surface would need to be: 2 2 21 2 22 2 5576 10 576 . 5 8 . 10 10 023 . 6 8 . 10 1 = (110)
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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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exam1spring2004 - ENSC 3313, Spring 2004 TEST 1 - KEY Name_...

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