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CLAB 267 Mid-Term

Terms in this set (25)

What is the purpose of the recrystallization process?
to purify solids
Which filtration technique will you use to separate a hot solution of benzoic acid from decolorizing carbon?
hot filtration (gravity)
Why is it necessary to use a hot solution of benzoic acid during the first filtration in this experiment?
it prevents premature crystallization and if benzoic acid solution becomes cold during filtrate it gets precipitated on the filter paper
4 criteria that must satisfy to be a solvent of choice for the recrystallization process of a solid
- Desired compound should be soluble in hot solvent and insoluble or nearly insoluble in a cold solvent
- Impurities should be insoluble in the solvent at all temperatures or must remain at least moderately soluble in the cold solvent
- Boiling point of the solvent has to be low enough to be readily removed from the crystals
- Solvent should not react chemically with the substance being purified
Why do you use boiling stones in this experiment?
it prevents bumping of the solution upon boiling
Two applications of the melting point of a compound in organic chemistry.
partial characterization and relative assessment of purity
What is melting point?
temperature at which the liquid and solid phases exist in equilibrium without a change in temperature
What are the two effects of impurities present in a substance while determining its melting point?
it decreases the melting point of a pure solid and has a wide melting range
How will you check the purity of your benzoic acid sample which you recovered in the last recrystallization experiment?
by determining its melting point; if we add additional heat to a "pure" substance it won't cause any spikes in temperature. Therefore if the solid has a fixed melting point it will determine the purity
What is eutectic mixture?
mixture of substances with a unique composition which exhibits a sharp melting range as though it were a pure compound -- a constant melting mixture
What is azeotrope?
a mixture that has a constant boiling point because the vapor has the same composition as the liquid mixture
Factors affecting observed melting point
-quantity of sample
-particle size/packing
-rate of heating
-chemical characteristics of sample
-purity of sample
Stationary phase and mobile phase used in the chromatography experiment
Stationary: alumina
Mobile: petroleum ether and dichloromethane
How will you determine purity of separated components of the mixture after column chromatography technique?
by using thin layer chromatography (TLC) to see if A & B contain spots indicating if its pure or impure
Rf value formula
distance the spot traveled / max distance eluent traveled
Which is more polar fluorene or 9-fluorenone? Why?
9-fluorenone is more polar because it has a carbon - oxygen double bond
Why should the level of the solvent in the chamber never reach the line where the mixture is to be separated during a TLC run?
because if it does it will cause the spots to smear and they will become useless; it will bring the chemical down into the chamber
What is boiling point?
temperature at which the vapor pressure of a liquid equals the pressure applied by the surroundings to the liquid
What effect would a reduction in atmospheric pressure have on the boiling point of ethyl acetate?
decreases the boiling point
Compare polarities based on their Rf values.
The lower the Rf value is the more polar the compound is
When distillation begins, which component of the mixture (having different boiling point) will be the greater component in the vapor phase and in the liquid phase? Explain
Greater in the vapor phase: the one with the high vapor pressure and the lower boiling point

Greater in the liquid: the one with the low vapor pressure and the higher boiling point because when a solution has the capabilities of being mixed together the compound with the lower boiling point converts into vapor more than the other
Why is it important that the cooling water in a distillation apparatus enter the condenser jacket at the lower end and exit at the upper?
because if it enters through the upper end it won't fill the jacket completely and it would effect its cooling efficiency
What is the purpose of aluminum foil in fractional distillation?
To keep heat in and decrease its condensation
How do you relate purity of fractional distillation to theoretical plate?
the greater number of theoretical plates is equal to the substances purity
What technique will you use to check purity of sample obtained from distillation?
gas chromatography
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