# HW3Solutions - 1 Problem W3.49 from Callister(8 points(a...

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MSE280: Introduction to Engineering Materials Homework 3: Solutions 1. (a) This portion of the problem asks that we specify which type of interstitial site the Be 2+ ions will occupy in BeO if the ionic radius of Be 2+ is 0.035 nm and the O 2- ions form an HCP arrangement. Since, from Table 3.4, r O 2- = 0.140 nm, then Problem W3.49 from Callister (8 points) r Be 2 + r O 2 = 0.035 nm 0.140 nm = 0.250 As r C / r A is between 0.225 and 0.414, the coordination number for Be 2+ is 4 (Table 3.3) therefore, tetrahedral interstitial positions are occupied. (b) We are now asked what fraction of these available interstitial sites are occupied by Be 2+ ions. Since there are two tetrahedral sites per O 2- ion, and the ratio of Be 2+ to O 2- is 1:1, one-half of these sites are occupied with Be 2+ ions. Problem 4.6 from Callister (8 points) We are asked to sketch portions of a linear polypropylene molecule for different configurations (using two-dimensional schematic sketches). (a) Syndiotactic polypropylene (b) Atactic polypropylene

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(c) Isotactic polypropylene Problem 4.9 from Callister (8 points) For an alternating copolymer that has a number-average molecular weight of 100,000 g/mol and a degree of polymerization of 2210, we are to determine one of the repeat unit types if the other type is ethylene. It is first necessary to calculate m using Equation 4.6 as m = M n DP = 100,000 g/mol 2210 = 42.25 g/mol Since this is an alternating copolymer we know that chain fraction of each repeat unit type is 0.5; that is f e = f x = 0.5, f e and f x being, respectively, the chain fractions of the ethylene and unknown repeat units. Also, the repeat unit molecular weight for ethylene is m s = 2( A C ) + 4( A H ) = 2(12.01 g/mol) + 4(1.008 g/mol) = 28.05 g/mol Now, using Equation 4.7, it is possible to calculate the repeat unit weight of the unknown
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HW3Solutions - 1 Problem W3.49 from Callister(8 points(a...

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