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Topic: Calculating Weight  (Read 3430 times)

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Offline biz

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Calculating Weight
« on: October 21, 2007, 07:29:48 PM »
I really need help with this question. Ugh.  ???

It's all about unit cancellation but I just can't seem to work it out! Please help.

"Calculate the weight of Na2S2O3 x 5H2O to prepare 400mL of a solution of ~4.0x10-2 M S2O32-"
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Offline agrobert

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Re: Calculating Weight
« Reply #1 on: October 21, 2007, 07:36:05 PM »
Cancel Molarity units multiplying by total volume.  As M = [x mol]/[1L].  So you need to convert 400-ml to L.  Then you have required mol needed so cancel out with the MW to get your answer in grams.  Remember one mol of Na2S2O3x5H2O gives 1 mol of S2O3-2.
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Offline biz

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Re: Calculating Weight
« Reply #2 on: October 21, 2007, 07:49:28 PM »
Here's what I did:

4.0 x 10-2 mol    x   0.4 L   =   0.016 mol
-------------------------      -------
             L                               1

So then, 0.016 mol x the molar mass of  Na2S2O3 x 5H2O? Is that right?
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Offline biz

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Re: Calculating Weight
« Reply #3 on: October 21, 2007, 07:58:16 PM »
Ok, yeah. I got it! Thanks for your help.
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