Chemical Forums
Chemistry Forums for Students => Undergraduate General Chemistry Forum => Topic started by: jjwill2000 on December 09, 2008, 11:44:13 AM
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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
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Nothing obvious, 7.5 mL should it be.
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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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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
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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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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.
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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!
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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.