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Homework 4 Solutions

# Homework 4 Solutions - HW 4 solutions 4.3 Below molecular...

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HW 4 – solutions 4.3 Below, molecular weight data for a polytetrafluoroethylene material are tabulated. Compute (a) the number- average molecular weight, (b) the weight-average molecular weight, and (c) the degree of polymerization. Molecular Weight Range ( g/mol ) x i w i 10,000–20,000 0.03 0.01 20,000–30,000 0.09 0.04 30,000–40,000 0.15 0.11 40,000–50,000 0.25 0.23 50,000–60,000 0.22 0.24 60,000–70,000 0.14 0.18 70,000–80,000 0.08 0.12 80,000–90,000 0.04 0.07 Solution (a) From the tabulated data, we are asked to compute , the number-average molecular weight. Using Equation 4.5a, this is computation is carried out below. Molecular wt Range Mean M i x i x i M i 10,000-20,000 15,000 0.03 450 20,000-30,000 25,000 0.09 2250 30,000-40,000 35,000 0.15 5250 40,000-50,000 45,000 0.25 11,250 50,000-60,000 55,000 0.22 12,100 60,000-70,000 65,000 0.14 9100 70,000-80,000 75,000 0.08 6000 80,000-90,000 85,000 0.04 3400 ____________________________

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(b) From the tabulated data, we are asked to compute , the weight-average molecular weight. Thus, using Equation 4.5b, this computation is carried out as follows: Molecular wt. Range Mean M i w i w i M i 10,000-20,000 15,000 0.01 150 20,000-30,000 25,000 0.04 1000 30,000-40,000 35,000 0.11 3850 40,000-50,000 45,000 0.23 10,350 50,000-60,000 55,000 0.24 13,200 60,000-70,000 65,000 0.18 11,700 70,000-80,000 75,000 0.12 9000 80,000-90,000 85,000 0.07 5950 ___________________________ (c) Now we are asked to compute the degree of polymerization, which is possible using Equation 4.6. For polytetrafluoroethylene, the repeat unit molecular weight is just m = 2( A C ) + 4( A F ) = (2)(12.01 g/mol) + (4)(19.00 g/mol) = 100.02 g/mol And
4.4

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Homework 4 Solutions - HW 4 solutions 4.3 Below molecular...

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