Chem Lab Exam II UPDATED
QUESTIONS AND CORRECT ANSWERS
Is Beer's law valid at wavelengths other than lambda max? - CORRECT ANSWER Yes
Lambda max was determined to be 810 nm for a copper (II) nitrate standard solution.
Following the experiment protocols, a wavelength of 640 nm was selected for the calibration
absorbance measurements.
Which wavelength is higher energy? - CORRECT ANSWER 640
A student prepares the dissolved brass solution by diluting to 100.0 mL mark using a
volumetric flask and thoroughly mixing. Before transferring solution into a cuvet their lab
partner accidentally spills about half the solution. How will the concentration of the unspilled
solution be affected? - CORRECT ANSWER Stay the same
The measured absorbance of their dissolved brass nail solution was 0.569. The brass nail
solution was diluted to 100.0 mL before absorbance was measured.
Based on their data, what is the concentration (in M) of copper(II) ions dissolved in this
solution? - CORRECT ANSWER 0.0546 M
If the concentration of copper is 0.0246 M and the volume of the solution is 100.0 mL,
calculate the mass of copper in solution. - CORRECT ANSWER 1.56×10^-1 g Cu2+
A 1.050 g sample of brass washers was reacted with excess nitric acid. Not all of the brass
dissolved.
At the end of the experiment there was 0.276 grams of unreacted brass solid remaining.
The calibration graph and absorbance indicated there was 0.565 grams of copper in the
solution.
What was the mass of brass that reacted (dissolved)? 0.774 g
What was the percent copper in the dissolved brass sample? 73.0% - CORRECT
ANSWER 0.774 g
, 73.0%
A student follows the procedure almost exactly but forgets to dry the undissolved brass before
measuring its mass. How will this affect the percent copper calculation? - CORRECT
ANSWER The calculated percent copper will be higher.
Which of these are valid reasons not to use lambda max? - CORRECT ANSWER The
procedure says to limit the absorbance to 1 or less.
The procedure says to only choose between 550 - 740 nm
The spectra is not smooth near lambda max making measurements unreliable.
There is a significant portion of the graph above that is below zero. This is due to a common
mistake made by spectroscopy novices.
What part of the procedural step was omitted or done incorrectly to generate this spectrum? -
CORRECT ANSWER Failing to calibrate the spectrometer
What is the unit of conductivity? - CORRECT ANSWER S/m (siemen/m)
When the concentration of a solution increases the conductivity decreases. - CORRECT
ANSWER False
When temperature of the solution decreases conductivity decrease .
If the solution were heated, conductivity should increase . - CORRECT
ANSWER decrease
increase
For each pair of solutions below choose which solution will have the higher conductivity:
0.01 M HCl (hydrochloric acid) or 0.01 M HCH3COO (acetic acid)? 0.01 M HCl
QUESTIONS AND CORRECT ANSWERS
Is Beer's law valid at wavelengths other than lambda max? - CORRECT ANSWER Yes
Lambda max was determined to be 810 nm for a copper (II) nitrate standard solution.
Following the experiment protocols, a wavelength of 640 nm was selected for the calibration
absorbance measurements.
Which wavelength is higher energy? - CORRECT ANSWER 640
A student prepares the dissolved brass solution by diluting to 100.0 mL mark using a
volumetric flask and thoroughly mixing. Before transferring solution into a cuvet their lab
partner accidentally spills about half the solution. How will the concentration of the unspilled
solution be affected? - CORRECT ANSWER Stay the same
The measured absorbance of their dissolved brass nail solution was 0.569. The brass nail
solution was diluted to 100.0 mL before absorbance was measured.
Based on their data, what is the concentration (in M) of copper(II) ions dissolved in this
solution? - CORRECT ANSWER 0.0546 M
If the concentration of copper is 0.0246 M and the volume of the solution is 100.0 mL,
calculate the mass of copper in solution. - CORRECT ANSWER 1.56×10^-1 g Cu2+
A 1.050 g sample of brass washers was reacted with excess nitric acid. Not all of the brass
dissolved.
At the end of the experiment there was 0.276 grams of unreacted brass solid remaining.
The calibration graph and absorbance indicated there was 0.565 grams of copper in the
solution.
What was the mass of brass that reacted (dissolved)? 0.774 g
What was the percent copper in the dissolved brass sample? 73.0% - CORRECT
ANSWER 0.774 g
, 73.0%
A student follows the procedure almost exactly but forgets to dry the undissolved brass before
measuring its mass. How will this affect the percent copper calculation? - CORRECT
ANSWER The calculated percent copper will be higher.
Which of these are valid reasons not to use lambda max? - CORRECT ANSWER The
procedure says to limit the absorbance to 1 or less.
The procedure says to only choose between 550 - 740 nm
The spectra is not smooth near lambda max making measurements unreliable.
There is a significant portion of the graph above that is below zero. This is due to a common
mistake made by spectroscopy novices.
What part of the procedural step was omitted or done incorrectly to generate this spectrum? -
CORRECT ANSWER Failing to calibrate the spectrometer
What is the unit of conductivity? - CORRECT ANSWER S/m (siemen/m)
When the concentration of a solution increases the conductivity decreases. - CORRECT
ANSWER False
When temperature of the solution decreases conductivity decrease .
If the solution were heated, conductivity should increase . - CORRECT
ANSWER decrease
increase
For each pair of solutions below choose which solution will have the higher conductivity:
0.01 M HCl (hydrochloric acid) or 0.01 M HCH3COO (acetic acid)? 0.01 M HCl
