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Author: Subject: What does this mean?
netnofeels
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[*] posted on 1-6-2019 at 13:17
What does this mean?

Im following some instructions and I really dont understand what this means. It says 'the residue was vacuum distilled, and the fraction boiling at 75-85°C/8mmHg weighed 90g'

The two compounds that should be in there have boiling points of around 200c and and I know its under vacuum but 8mmHg would do nothing. Im thinking that could be vapour pressure but im not sure and if it was i dont know why that would even matter. What does the 8mmHg mean and how would you be able to calculate the boiling point it should have under vacuum?
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horribilis
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[*] posted on 1-6-2019 at 13:33


Quote: Originally posted by netnofeels  
Im following some instructions and I really dont understand what this means. It says 'the residue was vacuum distilled, and the fraction boiling at 75-85°C/8mmHg weighed 90g'

The two compounds that should be in there have boiling points of around 200c and and I know its under vacuum but 8mmHg would do nothing. Im thinking that could be vapour pressure but im not sure and if it was i dont know why that would even matter. What does the 8mmHg mean and how would you be able to calculate the boiling point it should have under vacuum?


Why would the fact that it's under vacuum at 8 mm Hg do nothing? Plug it into a nomograph: https://www.sigmaaldrich.com/chemistry/solvents/learning-cen... A substance with a 200 C atmospheric boiling point should be ~ 72 C boiling point at 8 mm Hg vacuum. The units mm Hg are units for pressure.

[Edited on 1-6-2019 by horribilis]
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[*] posted on 1-6-2019 at 15:58


Quote: Originally posted by horribilis  


Why would the fact that it's under vacuum at 8 mm Hg do nothing? Plug it into a nomograph: https://www.sigmaaldrich.com/chemistry/solvents/learning-cen... A substance with a 200 C atmospheric boiling point should be ~ 72 C boiling point at 8 mm Hg vacuum. The units 8mm Hg are units for pressure.

[Edited on 1-6-2019 by horribilis]


Yes units of absolute pressure. I guess the OP was thinking 8mm below atmospheric pressure



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