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Author Topic: find chemical formula from compounds  (Read 702 times)

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heellosky

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find chemical formula from compounds
« on: December 16, 2018, 05:22:05 PM »

Hello everyone,
Please help with my problems
A question is a wood waste generally consists of 43% of cellulose(C6H10O5), 35% of hemicellulose(C5H10O5) and 22% of lignin(C31H34O11) and the molecule mass of C is 12g/mole, O is 16 g/mole and H is 1g/mole as we all know.
So what is the chemical formula for wood?


My working is :
No of mole of carbon in cellulose is 12*6/162  =0.44
No of mole of hydrogen in cellulose is 10/162=0.0617
No of mole of oxygen in cellulose is 16*5/162=0.493

No of mole of carbon in hemicellulose is 12*5/150  =0.4
No of mole of hydrogen in hemicellulose is 10/150=0.067
No of mole of oxygen in hemicellulose is 16*5/150=0.533


No of mole of carbon in lignin is 12*31/582=0.639
No of mole of hydrogen in lignin is 34/582=0.0584
No of mole of oxygen in lignin is 16*11/582=0.302

So I calculate the number of moles of each element in different compounds,but I don't know how to link with the percentage given and what is it related to the chemical formula of wood ????


thanks


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Borek

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Re: find chemical formula from compounds
« Reply #1 on: December 16, 2018, 09:45:56 PM »

In general there is no such thing as a chemical formula for something that is a complex mixture.

For some calculations we can approximate composition with a formula-like notation, but that's not the same.
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heellosky

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Re: find chemical formula from compounds
« Reply #2 on: December 16, 2018, 09:51:36 PM »

so what is that formula-like notation? how to get it?
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Enthalpy

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Re: find chemical formula from compounds
« Reply #3 on: December 17, 2018, 02:21:10 AM »

Sidenote A: it doesn't matter for the computation, but the cited compounds are macromolecules like (C6H10O5)n.

When given a % composition, you should always wonder if it's mass, molar or sometimes volume %. Here for macromolecules, whose number of units vary, it can only be mass %.

Sidenote B: the question doesn't mention water despite wood always contains some. Keep in mind, especially for wood but not only, to avoid future errors. Here it should be like "the dry part of typical wood consists of..."

So if you take for instance 1kg of wood (dry part of it), can you deduce amounts of constituents, atoms, and so on?

The resulting formula will contain non-integer indices, and you need to choose arbitrarily one index, for instance as "n", since only the proportions are known. Or if you prefer, "on molecule of wood" makes no sense. Just like some fuels can be written CnH1.98n.
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