OChem Lab Final Questions With Complete Solutions
I don't know which of my layers is water and which is my
organic phase, and I've forgotten what solvent I used!
Which of the following observations would tell me that my top
phase is my organic phase
- I add a few drops of water to the top of my separatory funnel.
The droplets mix with the top phase
- My top phase looks slightly cloudy
- I add a few drops of water to the top of my separatory funnel.
The droplets pass through the top phase without mixing, then
mix with the bottom phase
- My top phase is yellow colored
I add a few drops of water to the top of my separatory funnel.
The droplets pass through the top phase without mixing, then
mix with the bottom phase
- water passing through the top phase and mixing with the
bottom phase is an observation that definitely tells you that your
top phase is the organic phase, and the bottom phase is aqueous
Column Chromatography using silica gel separates compounds
by...
Polarity
As opposed to a TLC, the eluent moves ..... in a chromatography
column
downwards
,A flash column is faster than a normal gravity column thanks to
the application of
Air pressure
The construction of your column should include
- a layer of sand at the bottom of the column
- a layer of sand at the top of the column
- a short plug of cotton
- a layer of silica gel
all
Each time you add eluent, you should elute the solvent
completely out of the silica gel so it becomes dry again before
adding more eluent.
false! your solvent should never go underneath the top layer of
sand of your column.
When doing a gradient elution you start your column with a __
eluent mixture and progressively
____polarity until your target eluent mixture is reached
nonpolar
increase
When evaluating an eluent for use in a column by TLC, the Rf
of your desired compound should be...
0.3-0.5
Order these solvent mixtures from least polar (1) to most polar
(4)
9:1 Hexanes/Ethyl acetate
,5:2 Hexanes/Ethyl acetate
2:3 Ethyl acetate/Hexanes
1:1 Hexanes/Ethyl acetate
least: 9:1 Hexanes/Ethyl acetate
5:2 Hexanes/Ethyl acetate
2:3 Ethyl acetate/Hexanes
most: 1 Hexanes/Ethyl acetate
Plastic syringe containing silica gel and sand goes in
Solid waste
Paper towels soiled with ethyl acetate and hexanes goes in
solid waste
What property of caffeine could be making the acid-base
extraction less efficient than the column chromatography?
caffeine is somewhat soluble in aqueous solutions
- Caffeine is somewhat soluble in aqueous solutions, which is
why coffee and other caffeinated beverages exist! During your
acid base experiment, each aqueous extraction reduces the
amount of caffeine in your organic phase. A technique like
column chromatography that does not use aqueous solutions to
separate compounds will therefore be more efficient at
recovering caffeine.
true or false: In flash column chromatography, the silica gel is
the stationary phase and the eluent is the mobile phase.
true
, The cotton plug and bottom layer of sand in a column serves
what purpose?
- it prevents silica gel from flowing out of the column
- it prevents the solvent from disturbing the silica gel layer
- it helps load the mixtures on the column
- It creates a flat surface for the silica gel layer to rest on
- it prevents silica gel from flowing out of the column
- It creates a flat surface for the silica gel layer to rest on
If, during your experiment, instead of regular silica gel you had
used reverse phase silica gel, which compound would have
eluted out of the column the fastest?
- caffeine
- acetaminophen
- aspirin
- caffeine
Equilibration of the silica gel column:
- Removes air bubbles and cracks from the silica gel layer
- Can lead to heat being released
- Is achieved by cycling the most polar eluent through the
column
- Must be done before loading the sample mixture onto the
column
- Is essential for a good separation
- Can lead to heat being released
- Is achieved by cycling the most polar eluent through the
column
I don't know which of my layers is water and which is my
organic phase, and I've forgotten what solvent I used!
Which of the following observations would tell me that my top
phase is my organic phase
- I add a few drops of water to the top of my separatory funnel.
The droplets mix with the top phase
- My top phase looks slightly cloudy
- I add a few drops of water to the top of my separatory funnel.
The droplets pass through the top phase without mixing, then
mix with the bottom phase
- My top phase is yellow colored
I add a few drops of water to the top of my separatory funnel.
The droplets pass through the top phase without mixing, then
mix with the bottom phase
- water passing through the top phase and mixing with the
bottom phase is an observation that definitely tells you that your
top phase is the organic phase, and the bottom phase is aqueous
Column Chromatography using silica gel separates compounds
by...
Polarity
As opposed to a TLC, the eluent moves ..... in a chromatography
column
downwards
,A flash column is faster than a normal gravity column thanks to
the application of
Air pressure
The construction of your column should include
- a layer of sand at the bottom of the column
- a layer of sand at the top of the column
- a short plug of cotton
- a layer of silica gel
all
Each time you add eluent, you should elute the solvent
completely out of the silica gel so it becomes dry again before
adding more eluent.
false! your solvent should never go underneath the top layer of
sand of your column.
When doing a gradient elution you start your column with a __
eluent mixture and progressively
____polarity until your target eluent mixture is reached
nonpolar
increase
When evaluating an eluent for use in a column by TLC, the Rf
of your desired compound should be...
0.3-0.5
Order these solvent mixtures from least polar (1) to most polar
(4)
9:1 Hexanes/Ethyl acetate
,5:2 Hexanes/Ethyl acetate
2:3 Ethyl acetate/Hexanes
1:1 Hexanes/Ethyl acetate
least: 9:1 Hexanes/Ethyl acetate
5:2 Hexanes/Ethyl acetate
2:3 Ethyl acetate/Hexanes
most: 1 Hexanes/Ethyl acetate
Plastic syringe containing silica gel and sand goes in
Solid waste
Paper towels soiled with ethyl acetate and hexanes goes in
solid waste
What property of caffeine could be making the acid-base
extraction less efficient than the column chromatography?
caffeine is somewhat soluble in aqueous solutions
- Caffeine is somewhat soluble in aqueous solutions, which is
why coffee and other caffeinated beverages exist! During your
acid base experiment, each aqueous extraction reduces the
amount of caffeine in your organic phase. A technique like
column chromatography that does not use aqueous solutions to
separate compounds will therefore be more efficient at
recovering caffeine.
true or false: In flash column chromatography, the silica gel is
the stationary phase and the eluent is the mobile phase.
true
, The cotton plug and bottom layer of sand in a column serves
what purpose?
- it prevents silica gel from flowing out of the column
- it prevents the solvent from disturbing the silica gel layer
- it helps load the mixtures on the column
- It creates a flat surface for the silica gel layer to rest on
- it prevents silica gel from flowing out of the column
- It creates a flat surface for the silica gel layer to rest on
If, during your experiment, instead of regular silica gel you had
used reverse phase silica gel, which compound would have
eluted out of the column the fastest?
- caffeine
- acetaminophen
- aspirin
- caffeine
Equilibration of the silica gel column:
- Removes air bubbles and cracks from the silica gel layer
- Can lead to heat being released
- Is achieved by cycling the most polar eluent through the
column
- Must be done before loading the sample mixture onto the
column
- Is essential for a good separation
- Can lead to heat being released
- Is achieved by cycling the most polar eluent through the
column
