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Anonymous Poster

Determining Desiccant Mass

11/18/2009 1:52 PM

Hydrate is Na2CO3.7H2O

What mass of anhydrous Na2CO3 is required to dessicate (remove all the water) from an 80Kg body that is 78% water by mass?

How do you do this type of question?
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GKC
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#1

Re: How much anhydrous is needed to dessicate?

11/18/2009 2:21 PM

Start by determining the mass of water in the 80 Kg body. At 78%, the body has 62.4 Kg of water or 62,400 grams of water. Given the molecular weight of water (18.01528 g/mol), this is 3463.726 moles of water. Since Na2CO3 can absorb 7 moles of water per mole of Na2CO3, you need 494.818 moles of dry Na2CO3 to absorb all of the water (assuming 100% transfer efficeincy). 494.818 moles of Na2CO3 times the molecular weight of Na2CO3 gives 52,444.99 grams of Na2CO3 needed or 52.44499 Kg.

After accounting for significant figures, you need 52 Kg Na2CO3 to absorb all the water.

62.4Kg H2O
62,400g H2O
18.01528Mole Wt. H2O
3,463.726Moles H2O
494.818Moles Na2CO3 needed
105.9884Mole Wt. Na2CO3
52,444.99Grams Na2CO3 needed
52.44499Kg Na2CO3 needed
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Anonymous Poster
#2
In reply to #1

Re: How much anhydrous is needed to dessicate?

11/18/2009 2:29 PM

Thank you.

That is exactly what I got. The answer key says 100 Kg for some reason. I don't know why. Everything you say sounds good. But i think you're right.

Thanks again
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YesMAM
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#3
In reply to #2

Re: How much anhydrous is needed to dessicate?

11/19/2009 11:32 AM

Your analysis assumes 100% efficiency for the reaction. This is very seldom the case.

1. Reaction at the surface of your desiccant will be more efficient than inside your pile, or at bottom of pile.

2. Reaction at the surface will not likely be 100% either.

3. Your desiccant will also have to absorb water vapor out of the air inside your container. This will further increase your desiccant requirements.

Rule of thumb is about 2 times absorbant per unit of material. Not sure if this accounts for your difference.

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Doorman
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#4

Re: Determining Desiccant Mass

11/19/2009 4:37 PM

Just curious...

What are you doing... Making a mummy?

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