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Chemistry Forums for Students => Undergraduate General Chemistry Forum => Topic started by: Dugy on December 04, 2018, 07:43:51 PM

Title: Polymer weight calculations
Post by: Dugy on December 04, 2018, 07:43:51 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
Title: Re: Polymer weight calculations
Post by: mjc123 on December 05, 2018, 04:50:17 AM
You need to show some effort before being given help, that is a forum rule.

Do you know any equations for molecular weight averages?
Title: Re: Polymer weight calculations
Post by: Dugy on December 05, 2018, 05:19:01 AM
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.
Title: Re: Polymer weight calculations
Post by: Borek on December 05, 2018, 06:59:24 AM
Would assuming that you have 1000g sample of the polymer help?
Title: Re: Polymer weight calculations
Post by: mjc123 on December 05, 2018, 08:16:46 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