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Suggest an explanation and method of improvement when a reaction has negative yield i.e. the mass of container with product is less than the mass of empty container. This happened to me twice in an orgo lab and I have no idea whats going on.

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    $\begingroup$ There is some error in your measurement because this isn't possible. $\endgroup$ – jerepierre Mar 23 '16 at 1:01
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    $\begingroup$ Backfilled an empty flask with hydrogen? $\endgroup$ – SendersReagent Mar 23 '16 at 1:03
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    $\begingroup$ Error in calculation as this is not possible. Negative yield would be like saying you went out and destroyed someone else's product $\endgroup$ – StevieD Mar 23 '16 at 1:08
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    $\begingroup$ How large was the difference as compared to the weight of the container and/or the expected weight of the product? ? Did you use the same container? Back in the days, round-bottom flasks sometimes had little glass hooks attached to the neck (for springs). Did one of the hooks break off in between two measurements? Did you use a cap/lid on the empty container? $\endgroup$ – Klaus-Dieter Warzecha Mar 23 '16 at 11:26
  • $\begingroup$ In all seriousness, I've had to deal with negative yields before because I was weighing glassware off the shelf before concentrating down a very small amount of product on high-vac. I still had product by NMR, but it was very dilute despite being all I had. What I do now when I have very, very small amounts of product is pump on my flask before getting a tare weight to remove any moisture on the inside of the flask. That small amount of moisture evaporating was enough to throw it off, apparently, because it doesn't happen anymore. This is not likely to be what happened to you, though. $\endgroup$ – SendersReagent Mar 24 '16 at 5:29
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If the mass of the container with product is less than without then there are some possible mistakes:

  • you wrote down/used the wrong mass/container
  • you weighted the empty container with plug/screw cap and afterwards without
  • you changed the plug/screw cap
  • you had the container filled with a gas with higher density than air (common with argon), which can be a difference of some hundred mg depending on the size.
  • the temperature wasn't the same, don't weigh hot/warm flasks

From my experience as TA in synthetic labs: it's most likely the second one.

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    $\begingroup$ From my experience during my PhD student time, it’s likely the last. $\endgroup$ – Jan Nov 12 '16 at 16:19
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At the risk of stating the obvious, one possible explanation is the release of a volatile product. Thus the use of thionyl chloride to produce an acyl chloride releases Sulphur dioxide which release takes mass with it.

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    $\begingroup$ That is not a negative yield... that's called "I don't know how to calculate a theoretical yield". $\endgroup$ – orthocresol Mar 23 '16 at 19:15

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