Lab Quiz 1 Flashcards
Why do we call it melting range?
melting isn’t always one temp; when substances are mixed there is a range of melting
What can melting range tell you?
identity and purity
How do the melting ranges of pure compounds and mixtures differ?
pure compound = 2 degrees melting range
mixture = 5 degree melting range and lower temperature than pure
How is melting range determined?
heated at a controlled rate, then record when the substance starts melting and has become completely melted
How does recrystallization separate a compound from impurities?
differences of hot and cold solubility
What characteristics must a recrystallization solvent have?
dissolve compound of interest when hot but not when cold
Should the compound dissolve in hot solvent?
yes
How did you check the purity of your compound before and after the recrystallization?
we used melting point
How can two compounds be separated by distillation?
put in boiling flask, gradual increase not a huge jump, can’t cleanly separate two liquids
What is the ideal boiling point difference between compounds for a simple distillation?
at least 50 degrees C
Which distillation (simple or fractional) would separate 2 compounds with a boiling point difference of
25 degrees celsius?
fractional distillation
How did you perform a distillation?
boiling a liquid then condensing the vapor
Where does the water go in and out of the condenser?
it goes in at the bottom and exits at the top
What is the usual purpose of chromatography?
separation, small scale, tiny drop on the tlc plate, identity of the components
How does TLC separate compounds of similar structure?
depends if it sticks to the solid, depends on the compound
Should you handle the TLC plate by the edges or it does not matter?
you should handle the plate by the edges
How did you apply a sample to the plate, develop the chromatogram, and locate the separated
compounds?
apply sample -> capillary tube need small sample -> putting the plate in solvent above where compound was placed -> use UV light causing it to go except where the compound is located
What is Rf?
[Dc (distance compound travelled)] / Ds (distance the mobile phase travelled)]
What was the purpose of the extraction you did?
isolate caffeine from tea
You added sodium carbonate to the tea solution before extracting with chloroform. How was this
supposed to help the extraction?
tea contains tannins which dissolve in chloroform, don’t want tannins with tea so add sodium carbonate so not tannins in extraction. Na2CO3 (base) causes tannins to stay in water
What glassware item allows you to mix and then separate two immiscible liquids?
separatory funnel
After extracting the caffeine with chloroform, how did you remove this solvent (what apparatus and
what did it do)?
rotovap, flask on apparatus
- warms
- rotates
end with crude caffeine in flask
Experiment 1: Melting Point
technique: melting point
principle and physical properties: compound has a characteristic melting point and identifying a substance also shows an index of purity because of you melt a compound it will always be the same
procedure: fill melting point tube, then put in machine with small increments of heating to determine mp
Experiment 2: Recrystallization of benzoic acid
technique: recrystallization
principle and physical properties: temp dependence of solubility, differences is temperature dependent, if there is impurity when hot it dissolves, if cold impurities stay
procedure: dissolve crude in minimum amount of hot solvent -> let the sample cool -> crystals form and can be collected by vacuum filtration -> find melting range for purity
