Chem 112 Lab Practical Questions with
Verified Answers
Calculate the molality of a sugar solution that contains 3.94g sucrose (C12H22O11) dissolved
in 285 g water
3.94 g sucrose=0.0115 mol sucrose, 285g water=0.285kg water; 0.0115mol sucrose/0.285kg
water= 0.0404m
At -14.5C, what is the maximum mass of sucralose (C12H19Cl13O8) you can add to 1.50kg of
pure water and still have the solution freeze? Assume that sucralose is a molecular solid
and does not ionize when it dissolves in water.
Tf=Kfm, Kf H2O=1.86C/m (need to find the mass of sacralose needed to combine with 1.50kg
water) 14.5C= (1.86 C/m) x m, 14.5/1.86=7.80m (molality sucralose); (7.80mol/kg H2O)(1.50kg
H2O)=11.7 mol sucralose; (11.7mol)(397.63g)=4650g
percent error
[experimental value-actual value]/actual value x 100
Why when an ice storm is coming do many states spread salt on their roadways?
salt + ice depresses the usual freezing point of water , and increases the entropy of the water
(liquefying it, even at a lower temp); this causes less ice on the road because a lower
temperature is needed to solidify the ice
As a solution freezes, the solid that forms is essentially pure, frozen solute molecules
incorporated. With this in mind, explain why the freezing point of a solution slowly
decreases with time rather than stay constant as with the freezing point of a pure solvent.
,Imagine a container of pure solution. As this solution begins to freeze a pure frozen solute
forms, and the solvent becomes a more concentrated version of the original solution.The
change in entropy continues to increase (between the solid and liquid), and the temperature
required to balance the entropy decreases in the frozen solvent, and increases in the
surroundings in order to lower its temperature
What are the two ways light passing through a sample can be measured through
spectrophotometry?
Transmittance and Absorbance
How is transmittance found?
dividing light intensity that passes light through solution by the lighting that passes through
the blank sample; %T=I/I0 x 100
How is absorbance found?
amount of light absorbed by the sample; A=-logT
Beer's law
A=ℇbc; A=absorbance, ℇ= molar absorbitivity, b=path length, c=concentration
Important things to remember when conducting spectophotometry
a "blank" sample containing solvent must run 1st
Absorbance should be taken at max wavelength
Standard curve/calibration curve
data points of a solution w/ known concentration; allows researchers to determine
concentration of a sample w an unknown concentration by recording unknown's absorbances
and comparing them to the standard curve
what is the molar absorbtivity is the slope is y=76x
76=ℇ
, What is the focus of experiment 8?
Chlorophyll-a extracted from a spinach leaf in isopranol, and the conc. is found with
spectophotometry. The mg chlorophyll per g of spinach was calculated
Why is a blank sample measured first?
to remove the signal caused by the cuvette walls and solvent
Why must a cuvette be placed in the spectophotometer be placed in the same
orientation each time it is used?
The transmittance of the cuvette must be measured in the same place every time
Rank the colors in lowest to highest energy
ROYGBV
Highest energy photons have a ? frequency, and a ? wavelength
higher, lower
During a spectophotometric analysis of an unknown, you measure your unknown solution's
absorbance, and the value is higher than the highest standard you used ti make your
standard curve. What should you do so you can still use your standard curve to find the
unknown's concentration?
Dilute the unknown so that it will have an absorbance within the standard curve. Once the
diluted unknown concentration is determined, the full strength concentration can be
calculated if the dilution process is recorded. Beer's Law only applies to dilute solutions, so
diluting the unknown is better than making new standards.
A student fills a beaker w stock solution, then uses roughly half of the solution in
an experiment. After the lab is over, what should be done with the solution?
Disposed in correct waste container
Verified Answers
Calculate the molality of a sugar solution that contains 3.94g sucrose (C12H22O11) dissolved
in 285 g water
3.94 g sucrose=0.0115 mol sucrose, 285g water=0.285kg water; 0.0115mol sucrose/0.285kg
water= 0.0404m
At -14.5C, what is the maximum mass of sucralose (C12H19Cl13O8) you can add to 1.50kg of
pure water and still have the solution freeze? Assume that sucralose is a molecular solid
and does not ionize when it dissolves in water.
Tf=Kfm, Kf H2O=1.86C/m (need to find the mass of sacralose needed to combine with 1.50kg
water) 14.5C= (1.86 C/m) x m, 14.5/1.86=7.80m (molality sucralose); (7.80mol/kg H2O)(1.50kg
H2O)=11.7 mol sucralose; (11.7mol)(397.63g)=4650g
percent error
[experimental value-actual value]/actual value x 100
Why when an ice storm is coming do many states spread salt on their roadways?
salt + ice depresses the usual freezing point of water , and increases the entropy of the water
(liquefying it, even at a lower temp); this causes less ice on the road because a lower
temperature is needed to solidify the ice
As a solution freezes, the solid that forms is essentially pure, frozen solute molecules
incorporated. With this in mind, explain why the freezing point of a solution slowly
decreases with time rather than stay constant as with the freezing point of a pure solvent.
,Imagine a container of pure solution. As this solution begins to freeze a pure frozen solute
forms, and the solvent becomes a more concentrated version of the original solution.The
change in entropy continues to increase (between the solid and liquid), and the temperature
required to balance the entropy decreases in the frozen solvent, and increases in the
surroundings in order to lower its temperature
What are the two ways light passing through a sample can be measured through
spectrophotometry?
Transmittance and Absorbance
How is transmittance found?
dividing light intensity that passes light through solution by the lighting that passes through
the blank sample; %T=I/I0 x 100
How is absorbance found?
amount of light absorbed by the sample; A=-logT
Beer's law
A=ℇbc; A=absorbance, ℇ= molar absorbitivity, b=path length, c=concentration
Important things to remember when conducting spectophotometry
a "blank" sample containing solvent must run 1st
Absorbance should be taken at max wavelength
Standard curve/calibration curve
data points of a solution w/ known concentration; allows researchers to determine
concentration of a sample w an unknown concentration by recording unknown's absorbances
and comparing them to the standard curve
what is the molar absorbtivity is the slope is y=76x
76=ℇ
, What is the focus of experiment 8?
Chlorophyll-a extracted from a spinach leaf in isopranol, and the conc. is found with
spectophotometry. The mg chlorophyll per g of spinach was calculated
Why is a blank sample measured first?
to remove the signal caused by the cuvette walls and solvent
Why must a cuvette be placed in the spectophotometer be placed in the same
orientation each time it is used?
The transmittance of the cuvette must be measured in the same place every time
Rank the colors in lowest to highest energy
ROYGBV
Highest energy photons have a ? frequency, and a ? wavelength
higher, lower
During a spectophotometric analysis of an unknown, you measure your unknown solution's
absorbance, and the value is higher than the highest standard you used ti make your
standard curve. What should you do so you can still use your standard curve to find the
unknown's concentration?
Dilute the unknown so that it will have an absorbance within the standard curve. Once the
diluted unknown concentration is determined, the full strength concentration can be
calculated if the dilution process is recorded. Beer's Law only applies to dilute solutions, so
diluting the unknown is better than making new standards.
A student fills a beaker w stock solution, then uses roughly half of the solution in
an experiment. After the lab is over, what should be done with the solution?
Disposed in correct waste container
