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Author: Subject: Gas Generating Problems
CaptainOfSmug
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[*] posted on 15-3-2013 at 10:49
Gas Generating Problems


Okay, I know it says this forum isn't a text book substitute, but my professor hasn't gotten back to me and some of these problems are driving me bonkers...

So, for example:

Magnesium nitride decomposes in water and releases ammonia gas. If 335 mL of ammonia is produced at STP, what is the mass of magnesium nitride that reacted?

I know how to solve these problems I think by using PV=nRT and I'm solving for n in this case I believe. However, whats throwing me off is the STP part. Do I need to find the partial pressure of ammonia gas first and convert it to atmospheres? I know the rate is 0.0821 but would the temperature just be 273K? I have a hard time believing the gas is generated at room temperature...

Thanks in advance for the help! And I hope this goes without saying but please don't just give me the answer, I want to actually be able to solve these and a nudge in the right direction would help me greatly!

[Edited on 15-3-2013 by CaptainOfSmug]
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DraconicAcid
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[*] posted on 15-3-2013 at 10:53


STP is standard temperature and pressure. It doesn't matter what temperature the gas was generated at, it's heated or cooled (and expanded or pressurized) until it's at STP and the volume is measured.



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MrHomeScientist
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[*] posted on 15-3-2013 at 11:36


The way I would go about it is first determine n from the gas law, as you said. Then, find the equation for magnesium nitride's reaction with water. You can use that and the results from the first step to get to your answer. Good luck!

And kudos for not asking for spoonfeeding! There seems to be quite a bit of that lately. That might get you your answer short term, but you won't get far if you don't actually understand where the answer came from. The best way to understand is to do it yourself!
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CaptainOfSmug
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[*] posted on 15-3-2013 at 11:52


Okay thanks much! So I figured the equation is Mg3N2 + 6H2O --> Mg(OH)2 + 2NH3

So, would my equation essentially be (1)(.335)=n(0.0821)(273) so n =0.015 moles of NH3?
So the mass would then be 0.757g of magnesium nitride?

This seems logical to me and makes sense but just double checking. Thanks again for the help!
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MrHomeScientist
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[*] posted on 15-3-2013 at 12:35


Looks correct to me! It looks like you accounted for it in the math, but forgot to include the coefficient of 3 on the Mg(OH)2 in your post.
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DraconicAcid
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[*] posted on 15-3-2013 at 12:39


Quote: Originally posted by MrHomeScientist  
Looks correct to me! It looks like you accounted for it in the math, but forgot to include the coefficient of 3 on the Mg(OH)2 in your post.

Doesn't matter- you only use the NH3/Mg3N2 coefficient in the calculation.




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MrHomeScientist
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[*] posted on 15-3-2013 at 12:54


Quote: Originally posted by DraconicAcid  
Quote: Originally posted by MrHomeScientist  
Looks correct to me! It looks like you accounted for it in the math, but forgot to include the coefficient of 3 on the Mg(OH)2 in your post.

Doesn't matter- you only use the NH3/Mg3N2 coefficient in the calculation.


Ah, well... of course :D
Just making sure the full equation was correct!
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