# FA14 3313 Qz01 key

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• ENSC 3313, Fall 2014 Quiz 1 Name________________________________ Answer any 4 questions only, 1 point each 1. Tritium is used in all modern American nuclear weapons, but needs to be periodically replaced because

it has a half-life of 12.3 years. The US used to make tritium in up to 14 nuclear reactors all of which have closed down since 1988. They will need new supplies after 2011. Congress wishes to build a new reactor in South Carolina. Assuming tritium was 'made' in 1988, what proportion has decayed by the year 2004?

= t

o

t eAA

Substituting, 0.5 = e12.3/ = 17.75 Repeating At

A o = e16/17.75 = 0.41

0.41 left, hence 0.59 decayed (59%) 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 = 3 108 m/s and Planck's constant is h = 4.13 10-15 eVs.

The energy of a photon is given by: eV48.210500

1031013.4chhE 9815

=

=

==

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. 3. Write the electron configuration for Ca (atomic number 20) and determine the charge on a Ca ion in the

compound gypsum (CaSO42H2O). You may use the noble gas abbreviated version. Ca has atomic number 20, so we can write: 1s22s22p63s23p64s2 or alternatively, [Ar]4s2 Ca is a group IIA metal and will give up its electrons to form cations with a +2 charge.

• 4. How large is the space (in nm) between iron atoms along the edge of its body centered cubic unit cell? Note: the atomic radius of iron is 1.24 .

ANSWER The side length of a FCC unit cell is: a = 2 2 R = 2 2 0.128 a = 0.362nm Vcell = a3 = 0.3623 Vcell = 0.047nm3 5. The mass of a small diamond is 3.3 mg. How many C13 atoms are present if carbon contains 1.1 a/o of that

isotope? Avogadros number is 0.6022 1024. AMU of C = 12.011.

Number of C atoms = ( )( )

2024 1065.1106022.0/011.12

0033.0 =

Number of C13 atoms = (1.65 1020)(0.011) = 1.82 1018

6. In this course, we are very much concerned with structure-properties relationships. Use graphite and

diamond to illustrate this theme! Graphite has a hexagonal structure, hence it has very directional properties; it is essentially metal like in 2 dimensions (the basal plane) and a nonmetal in the c direction. When the crystallites are aligned in the basal plane, graphite (for example) is strong and stiff and serves as the reinforcement in graphite-epoxy composites. Diamond has a cubic structure, so it is isotropic

and has very different and nondirectional properties.