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Topic: actual yield is way off  (Read 4766 times)

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Offline jjwill2000

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actual yield is way off
« on: December 09, 2008, 11:44:13 AM »
Hello,

Can someone help with why my actual yield is so far off?

I combined .00813g of Mg ribbon with excess HCL and that produced 17mL of H2 gas.

Theoretically, I should have gotten 7.48 mL.

Mg + 2HCl --> H2 + MgCl2

I didn't run this reaction at STP .. but that should only account for less than 20% of the error.  I double checked the mass.  Can anyone see something obvious that I'm missing???

Thanks,
Josh

Offline Borek

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Re: actual yield is way off
« Reply #1 on: December 09, 2008, 01:45:35 PM »
Nothing obvious, 7.5 mL should it be.
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Offline P

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Re: actual yield is way off
« Reply #2 on: December 10, 2008, 04:13:18 AM »
how did you collect the H2?  Are you sure that all the 17 ml was H2 and not H2 mixed with air?
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Offline jjwill2000

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Re: actual yield is way off
« Reply #3 on: December 10, 2008, 09:57:38 AM »
Nope . no air was allowed in.  I used a graduated cylinder and all of the gas was from bubbles produced from this reaction.  I can't for the life of me figure out why I'm off by soooo much.

Josh

Offline P

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Re: actual yield is way off
« Reply #4 on: December 11, 2008, 04:23:20 AM »
OK - clutching at straws now then  -  but how smooth is the plunger action in the gas collector?  Could it be a bit sticky, causing the pressure to build -  the plunger might then 'spring' out past the correct point and then stick there before drawing back under the reduced pressure..  ???
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Offline jjwill2000

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Re: actual yield is way off
« Reply #5 on: December 11, 2008, 01:36:09 PM »
In typing a response, I think I just figured out my problem.  I forgot to adjust the height of the liquid in the graduated cylinder with the height of the liquid in the beaker . thus equalizing the gas pressure in the cylinder with the atmospheric pressure outside the cylinder!  When I measured the volume of gas collected, the level of the liquid in the cylinder was much higher than the level of water in the beaker.  Man . I spent several days really confused . because of a stupid mistake.



Offline enahs

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Re: actual yield is way off
« Reply #6 on: December 11, 2008, 05:02:51 PM »
Mistakes happen, sometimes they are good actually. But that is why you write down and diagram everything you do in a lab! So you can figure out what went wrong later!

Offline Borek

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Re: actual yield is way off
« Reply #7 on: December 11, 2008, 06:52:35 PM »
In typing a response, I think I just figured out my problem.  I forgot to adjust the height of the liquid in the graduated cylinder with the height of the liquid in the beaker . thus equalizing the gas pressure in the cylinder with the atmospheric pressure outside the cylinder!  When I measured the volume of gas collected, the level of the liquid in the cylinder was much higher than the level of water in the beaker.  Man . I spent several days really confused . because of a stupid mistake.

That's not it. To get that large difference in volumes you will need much larger difference in pressures that you can get with half meter of water - and I doubt the difference between beaker and cylinder was that much.
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