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Topic: Stoichiometry with hydrates?  (Read 19019 times)

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Offline inspiration100

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Stoichiometry with hydrates?
« on: January 20, 2008, 11:59:06 AM »
Hi guys!  I have been assigned a question which has stoichiometry with a hydrate and I'm really unsure exactly how to do it.  How does a hydrate differ from another substance in stoichiometry?

The question:
I have 500.0 mL of 0.080 M Na2S2O3 and I need to know how many grams of sodium thiosulfate pentahydrage (Na2S2O3*5H2O) are needed to make up the solution above.

What's throwing me off is the hydrate, since I don't know how to add up the molar mass of a hydrate, or how to use one in stoichiometry.

My work so far:
500.0 mL x (10-3/1 mL) x (0.080 mol Na2S2O3)

My question is... what's next?  Please show work so I can do these in the future if you can   :D.
Thanks guys!
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Offline Kryolith

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Re: Stoichiometry with hydrates?
« Reply #1 on: January 20, 2008, 12:06:27 PM »
You calculated the amount of substance n=0.04 mol. So this is the amount of thiosulfate ions that is necessary.Thus you need 0.04 mol of your hydrate. With the molar mass of the hydrate you can calculate the mass of it.
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Offline inspiration100

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Re: Stoichiometry with hydrates?
« Reply #2 on: January 20, 2008, 12:19:07 PM »
Thanks.  Can I calculate the molar mass of the hydrate, or does that need to be given?
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Offline Kryolith

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Re: Stoichiometry with hydrates?
« Reply #3 on: January 20, 2008, 12:20:41 PM »
You need a periodic table (or anything else) to look up the mass of each element. Then you can calculate the molar mass of the compound.
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Offline inspiration100

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Re: Stoichiometry with hydrates?
« Reply #4 on: January 20, 2008, 12:30:25 PM »
Yes, but where i'm confused is how do I calculate the molar mass of the hydrate with the 5H2O attached on it?  Do I find the molar mas of H2O (18.015 grams/mole) times it by 5 and add it to the molar mass Na2S2O3?  Or does a hydrate require something else?

Thanks!
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Offline Borek

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Re: Stoichiometry with hydrates?
« Reply #5 on: January 20, 2008, 12:54:30 PM »
Quote from: inspiration100 on January 20, 2008, 12:30:25 PM
how do I calculate the molar mass of the hydrate with the 5H2O attached on it?  Do I find the molar mas of H2O (18.015 grams/mole) times it by 5 and add it to the molar mass Na2S2O3?

Yes.
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Offline inspiration100

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Re: Stoichiometry with hydrates?
« Reply #6 on: January 20, 2008, 03:48:32 PM »
Thanks, yes that appeared to work   8).  I come out with an answer around 9.929 grams.  If any of you chem geeks have some free time to check if I did it right, that would be much appreciated.  The molar mass of the hydrate = 248.22 grams to save some time.

Thanks all!
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Offline Kryolith

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Re: Stoichiometry with hydrates?
« Reply #7 on: January 20, 2008, 04:00:57 PM »
Your answer is correct. Good job!

You need to fill up 9.93 g hydrate with water to a volume of 500 ml.
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