1|Page
Chem 123 Lab Final exam questions and answers already graded
A+
In general, the smaller the melting-range, the higher the purity of the sample. - answer..,
PURE SUBSTANCE - answer..,NARROW RANGE AND EXTREMELY CLOSE TO
MELTING POINT (MIGHT BE A TINY BIT LESS)
impure substance - answer..,wide range and melts at a temperature much lower than the melting
point. if the melting point range is way higher than indicated, a sign that it is another molecule all
together.
why is ammonium hydroxide added in the sepratory funnel with the dcm and cola? - answer..,to
make phosphoric acid more soluble in water than the organic solvent is to add ammonium
hydroxide. a weak acid's salt is more soluble in water than the acid itself. want to separate the
phosphoric acid from the crude caffeine mixture.
Why must the filter paper be wet with DCM? - answer..,If the filter paper is soaked in DCM, NO
residual water pass through the filter paper.
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Fluted filter-paper greatly increases the efficiency of filtration, because the liquid only has to
passthrough one layer of paper at all points and runs freely down the side of the paper inside of
the filtration funnel - answer..,
rotary evaporators - answer..,used to remove solvents from a solution through evaporation. The
solvent DCM will evaporate and you're left with the solid (crude caffeine) on the inner walls of
the round bottom flask.
Why does the vacuum sublimation apparatus have to be dry? - answer..,as the solid evaporates,
pure caffeine is deposited on the outside of the cold finger via condensation. This material is the
sublimate. Must be dry or the caffeine will slip off of the cold finger and will not be able to
accumulate on the cold finger.
Sublimation - answer..,a separation technique used to purify a solid chemical substance. It is
commonly called vacuum sublimation. After sublimation, you can check your product's purity by
using melting point analysis.
Melting point analysis can also provide information about the purity of a sample. A substance
containing impurities usually melts at a lower temperature than the pure compound, and melts
over a wide range of temperatures. In general, the smaller the range of melting temperatures, the
higher the purity of the sample. (helps with the identification of a compound). - answer..,
k' = k [bleach] - answer..,
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Exp 11 Calculations - answer..,to calculate initial concentration of bleach or dye:
(9.00mL dye * (3.40e-5))/(10mL) =
(1.00 mL bleach * (0.090M))/(10mL)=
get k:
take the k' value and divide by the initial bleach concentration found above
An unknown sample of Yellow 6 solution has the absorbance equal to 0.91. The slope of the
Beer's law calibration plot constructed using a 3.8E-5M stock solution of Yellow 6 is 0.0117(%)-
1. What is the molar concentration of the unknown sample (in mole L-1)?Please give your
answer to 3 significant figures (1 additional SF is used to minimize rounding errors). -
answer..,abs/ concentration - slope
0.91/(0.0117)= concentration = 77.78%
0.7778 * stock dye concentration
Calculate the instantaneous reaction rates at 3 seconds and 9 seconds time for the pseudo first
order reaction of Yellow 6 with bleach, in which k' = 0.12 s-1. and slope = 0.0117 %^-1 -
answer..,1. convert slope to M, times by 100% and divide by the stock concentration of the
bottle. (concentration of the dye at 100%).(want M^-1)
Chem 123 Lab Final exam questions and answers already graded
A+
In general, the smaller the melting-range, the higher the purity of the sample. - answer..,
PURE SUBSTANCE - answer..,NARROW RANGE AND EXTREMELY CLOSE TO
MELTING POINT (MIGHT BE A TINY BIT LESS)
impure substance - answer..,wide range and melts at a temperature much lower than the melting
point. if the melting point range is way higher than indicated, a sign that it is another molecule all
together.
why is ammonium hydroxide added in the sepratory funnel with the dcm and cola? - answer..,to
make phosphoric acid more soluble in water than the organic solvent is to add ammonium
hydroxide. a weak acid's salt is more soluble in water than the acid itself. want to separate the
phosphoric acid from the crude caffeine mixture.
Why must the filter paper be wet with DCM? - answer..,If the filter paper is soaked in DCM, NO
residual water pass through the filter paper.
,2|Page
Fluted filter-paper greatly increases the efficiency of filtration, because the liquid only has to
passthrough one layer of paper at all points and runs freely down the side of the paper inside of
the filtration funnel - answer..,
rotary evaporators - answer..,used to remove solvents from a solution through evaporation. The
solvent DCM will evaporate and you're left with the solid (crude caffeine) on the inner walls of
the round bottom flask.
Why does the vacuum sublimation apparatus have to be dry? - answer..,as the solid evaporates,
pure caffeine is deposited on the outside of the cold finger via condensation. This material is the
sublimate. Must be dry or the caffeine will slip off of the cold finger and will not be able to
accumulate on the cold finger.
Sublimation - answer..,a separation technique used to purify a solid chemical substance. It is
commonly called vacuum sublimation. After sublimation, you can check your product's purity by
using melting point analysis.
Melting point analysis can also provide information about the purity of a sample. A substance
containing impurities usually melts at a lower temperature than the pure compound, and melts
over a wide range of temperatures. In general, the smaller the range of melting temperatures, the
higher the purity of the sample. (helps with the identification of a compound). - answer..,
k' = k [bleach] - answer..,
, 3|Page
Exp 11 Calculations - answer..,to calculate initial concentration of bleach or dye:
(9.00mL dye * (3.40e-5))/(10mL) =
(1.00 mL bleach * (0.090M))/(10mL)=
get k:
take the k' value and divide by the initial bleach concentration found above
An unknown sample of Yellow 6 solution has the absorbance equal to 0.91. The slope of the
Beer's law calibration plot constructed using a 3.8E-5M stock solution of Yellow 6 is 0.0117(%)-
1. What is the molar concentration of the unknown sample (in mole L-1)?Please give your
answer to 3 significant figures (1 additional SF is used to minimize rounding errors). -
answer..,abs/ concentration - slope
0.91/(0.0117)= concentration = 77.78%
0.7778 * stock dye concentration
Calculate the instantaneous reaction rates at 3 seconds and 9 seconds time for the pseudo first
order reaction of Yellow 6 with bleach, in which k' = 0.12 s-1. and slope = 0.0117 %^-1 -
answer..,1. convert slope to M, times by 100% and divide by the stock concentration of the
bottle. (concentration of the dye at 100%).(want M^-1)
