Chemical Forums
Chemistry Forums for Students => Undergraduate General Chemistry Forum => Topic started by: Dugy on December 04, 2018, 07:43:51 PM
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Need to work out number average and weight average of a polymer that contains two different chains. Only weight fraction and the mass of each chain is given. How can I work it out?
Mass one: 2000 g mol-1
Weight fraction: 0.5
Mass two: 250000 g mol-1
Weight fraction: 0.5
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You need to show some effort before being given help, that is a forum rule.
Do you know any equations for molecular weight averages?
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I know these equations:
Number average = ΣiNiMi/ΣiNi
Weight average = ΣiNiMi2/ΣiNiMi
Where Ni is number of chains and Mi is the mass.
However the number of chains is not given.
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Would assuming that you have 1000g sample of the polymer help?
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Your equations, in terms of number fractions, are correct, but in your case equations in terms of weight fractions would be more useful.
The mass of species i is NiMi, and its weight fraction Wi is NiMi/ΣNiMi
The average equations can then be rewritten as
Number average = ΣWi/Σ(Wi/Mi)
Weight average = ΣWiMi/ΣWi