Purification of Benzoic Acid Flashcards
why is it important to completely dry a solid before measuring the melting point?
water-soluble impurities will depress the melting point and widen the range
give 6 reasons why water is a good choice for recrystallization of benzoic acid
- lowers ba solubility when cold, raises when hot
- opposite for impurities, reducing amount in final product
- cheap
- readily available
- inert
- non-toxic
why use hot gravity filtration? (2)
- heat removes impurities of decolourizing carbon
2. also prevents recrystallization
why use flute filter paper? (2)
the folds allow quick passage of liquid and increase surface area to collect impurities
why is a 5-15% excess of solvent used?
to get correct range to prevent premature crystallization
what happens if volume is out of range?
must be condensed
why was the melting point of the ba measured?
as a measure of purity by comparing it to a known pure value, and to determine an unknown
why use an EF for recrystallization at the beginning of the lab?
more likely to lose solvent; crystals will condense on the neck and fall back in
what is the role of activated charcoal?
removes impurities
after hot flute gravity filtration, is the filtrate cooled fast or slowly? Why?
slowly; it promotes more crystallization
why is an ice-bath used?
cold temperature reduces solubility and increases yield
what are the sources of error for this lab?
- transfer loss
- loss to charcoal
- not all crystals can be removed from filtrate
what transfer is resposible for the majority of transfer loss? How can it be minimized?
hot flute paper. Reduce by keeping funnel as hot as possible (double layer funnel with hot water running through)
how can loss of crystals to filtrate be minimized? (2)
- reducing volume before crystallization
2. keeping filtrate as cold as possible
