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For my magnesium oxide lab, one of the question asks why I need to include the cover of the crucible when weighing it with the magnesium before the reaction, then again after with magnesium oxide, and in the beginning when its empty. Does it change if I don't include the cover? anyone have any hints or tips as to why I need to include the cover?

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When you burn magnesium it sputters. Without a cover bits would be thrown out. With the cover bits stick to the inside of the cover.

(1) Weight(crucible+lid+magnesium) - Weight(crucible+lid) = Weight(magnesium)

(2) Weight(crucible+lid+magnesium oxide) - Weight(crucible+lid+magnesium) = Weight(oxygen)

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  • $\begingroup$ The question is asking why you want to cover when weighing it. Not during the experiment $\endgroup$ – ChemPsycho Jan 16 '18 at 23:12
  • $\begingroup$ Think... if there is magnesium oxide on the inside of the cover from burning, what happens if you ignore the cover in the initial and final weighings? $\endgroup$ – MaxW Jan 17 '18 at 0:47
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Magnesium is going to be covered with a thin layer of $\ce{MgO}$ and is not really an issue here, but for calcined $\ce{MgO}$ you need to avoid both physical absorption and chemisorption of water vapors and $\ce{CO2}$ from the air:

$$ \begin{align} \ce{MgO + H2O &-> Mg(OH)2}\\ \ce{MgO + CO2 &-> MgCO3} \end{align} $$

Both magnesium hydroxide and carbonate are also hygroscopic and form a wide range of hydrates when kept in humid environment. It is necessary either to weigh the sample very quickly, or use the crucible with the lid tightly closed.

Ideally for such measurements you want to use a weighing bottle specifically designed for precise mass determination of hygroscopic and reactive solids:

Weighing bottles of various sizes
(Source)

In both cases make sure to tare the balance with both masses of the vessel and the lid beforehand in order to subtract those values from the experimental data. Of course, if you label the vessels, this also should be done prior to weighting when the crucible is cooled down to the room temperature.

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  • $\begingroup$ How does the water vapor and CO2 cause the need to weigh the crucible with the lid closed? Does it not work if you dont measure it with the lid? $\endgroup$ – ChemPsycho Jan 16 '18 at 22:17
  • $\begingroup$ Baloney. The crucible lid is not air tight. $\endgroup$ – MaxW Jan 16 '18 at 22:20
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    $\begingroup$ Why would u want this to be air tight. Oxygen has to go in to mix with magnesium $\endgroup$ – ChemPsycho Jan 16 '18 at 22:37
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    $\begingroup$ @ChemPsycho When you weigh substances, you do not want stuff to react with anything. $\endgroup$ – andselisk Jan 16 '18 at 22:38
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    $\begingroup$ @ChemPsycho your question is unclear. The way I interpret it is that you are trying to weigh the MgO product which you already obtained from a reaction previously. However, your comments here make no sense in that context, so I guess I was wrong? The problem is that nobody can accurately guess at what you mean by "my magnesium oxide lab", as nobody has your lab book with them. Would you care to elaborate by editing your question? $\endgroup$ – orthocresol Jan 16 '18 at 22:53

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