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MIDTERM_1_Fall2007_SOLUTIONS

# MIDTERM_1_Fall2007_SOLUTIONS - MECT 3345 MIDTERM...

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MECT 3345 MIDTERM EXAMINATION #1 10/03/07 PRINT NAME: ________________________________________________ Instructions: Only pen, calculator and these sheets allowed on desk. Relevant handouts are attached at the end. Time limit 1 hour 30 minutes. Attempt to solve all problems. 1

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CONSTANTS: Avogadro’s Number: N A = 6.022 x 10 23 Boltzmann Constant: k = 1.38 x 10 -23 J/K = 8.617 x 10 -5 eV/K Temperature Conversion: T [K] T[ o C] + 273 Problem 1 (15 points) a.) Define the meanings of atomic and mass numbers (3pt) b.) Explain the difference between atomic mass, averaged atomic mass and atomic weight (3pt) c.) Convert the atomic weight of Zn into units of g/mol (atomic weight of Zn is 65.41) (3pt) d.) Write down electron the configurations of F and Si (3pt) e.) What do all the elements in group VIIA have in common? (3pt) a.) Atomic number = number of protons in a nucleus. Mass number = sum of number of protons and neutrons in a nucleus. b.) atomic mass – mass of a particular atom (isotope). For example, atomic mass of 12 C, atomic mass of 13 C, etc. averaged atomic mass – averaged mass for all isotopes of a particular element. Averaging is based on the natural abundance of each isotopes. Atomic weight – synonymous with averaged atomic mass. c.) 65.41 amu/atom = 65.41 g/mol d.) From the periodic table: atomic numbers for F and Si are 9 and 14, respectively. F: 1s 2 2s 2 2p 5 Si: 1s 2 2s 2 2p 6 3s 2 3p 2 e.) All of them need one electron to form a closed shell. Alternatively, all of them have 5 electrons in the p shell with the highest principal quantum number. Problem 2 (15 points) 2
Consider an ionic pair X + Y - : the attractive and repulsive potential energies between them can be expressed as E A =-A/r and E R =B/r n , respectively. a.) derive the expressions for attractive and repulsive forces between the two ions (3pt) b.) if it is known that the equilibrium separation between the ions is r o =0.2 nm, that the bonding energy is E B = -6.1 eV and that n=10, determine the values of constants A and B. Also write down the units you obtain for each constant. (12pt) a.) dr dE F =

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MIDTERM_1_Fall2007_SOLUTIONS - MECT 3345 MIDTERM...

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