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Author Topic: Polymer weight calculations  (Read 585 times)

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Dugy

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Polymer weight calculations
« on: December 04, 2018, 01:43:51 PM »

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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mjc123

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Re: Polymer weight calculations
« Reply #1 on: December 04, 2018, 10:50:17 PM »

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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Dugy

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Re: Polymer weight calculations
« Reply #2 on: December 04, 2018, 11:19:01 PM »

I know these equations:

Number average = ΣiNiMiiNi

Weight average = ΣiNiMi2iNiMi

Where Ni is number of chains and Mi is the mass.

However the number of chains is not given.
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Borek

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Re: Polymer weight calculations
« Reply #3 on: December 05, 2018, 12:59:24 AM »

Would assuming that you have 1000g sample of the polymer help?
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mjc123

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Re: Polymer weight calculations
« Reply #4 on: December 05, 2018, 02:16:46 AM »

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
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