Basic Laboratory Skills Exam
Introduction
As part of the assessment plan for the biochemistry department, a basic laboratory skills
exam is given following all chemistry 105L and 106L sections. We anticipate that students will
be able to use their knowledge from the general chemistry sequence to be able to perform skills
that are determined as basic. These final practical exams are administered separately in all
chemistry 105L and 106L sections of Dr. Mark Cannon, Dr. Georgi Lukov and Dr. Daniel Scott.
The following problem was given. The students had ~60 minutes to plan and execute their
experiment and 30 minutes to generate a report on the experiment. The students were in the lab
with the computers so that they could do their data analysis using the software available on the
computer. The following is an example of a practical exam from both 105L and 106L.
CHEM 105L Practical Exam.
1) Make100.0 mL of a 0.100 M copper (II) sulfate solution using the provided salt. Write down how many
grams of the solid you use.
______________________________g
2) Determine the absorbance of the solution you made at 800 nm using the spectrophotometer and record the
absorbance value below. Then submit this sample to your instructor. Be sure to use a blank.
Absorbance at 800nm of the solution made in step 1 =______________________________
3) Given the data to the right, plot a standard curve and determine the concentration of copper (II) sulfate in
Unknown 1 provided by your instructor.
Molar
Concentration of copper (II) sulfate in Unknown 1
_____________________________
4) According to the solubility rules almost all metal hydroxides are
insoluble. This is also the case with copper(II) hydroxide --Cu(OH)2.
When copper(II) sulfate reacts with sodium hydroxide according to
the following balanced equation,
Absorbance at
800 nm
1.168
0.812
0.706
0.577
0.346
0.227
0.118
Concentration of
copper (II)
sulfate
0.100
0.070
0.060
0.050
0.030
0.020
0.010
CuSO4(aq) + 2NaOH(aq)  Cu(OH)2(s) + Na2SO4(aq)
copper(II) hydroxide precipitates out removing Cu +2 and OH- from
solution into the precipitated form. Because pH is related to the
concentration of OH- in solution, it is possible to titrate NaOH with CuSO4. As CuSO4 is added to NaOH
each Cu+2 removes two OH- ions from solution by precipitation (as seen in the equation above). This
process lowers the concentration of OH- in solution. When all the OH- ion is completely precipitated the pH
of the solution goes down.
You will titrate a solution with an unknown concentration of NaOH (Unknown 2…it is a solution) with
Table 1. Data used to make a standard curve for the
your CuSO4 solution from part 1. You will place your CuSO 4
concentration of copper sulfate relative to the
solution from part 1 into a burette and carefully use it to titrate a
absorbance at 800 nm.
solution of ~20 mL of water and somewhere between 2-10 mL of
Unknown 2. Again, in the burette you will place your CuSO 4 solution
from part 1, and into a flask you will place ~20 mL of water and Unknown 2. Because the pH will decrease
from a high number (~pH 12) to near pH 7 as the OH - is precipitated out of solution, we can use
phenolphthalein as the indicator. The phenolphthalein will turn from pink to colorless at the equivalence
point leaving behind the light blue of the CuSO4 that was just added. Again the color change will be a
pinkish purple to the color of the CuSO4 solution. At this point all the OH- is precipitated out and the moles
of CuSO4 used will be equal to half as many moles of NaOH that were in the flask to start with (see the
chemical equation). Repeat this titration until you are confident of the concentration of the unknown and
then record the unknown concentration below.
Concentration of NaOH in Unknown 2
_____________________________
5) Write an abstract, on the back of this page or stapled to it, for the work you did, and a short results section
showing any calculations and explaining how you know the values for Unknown 1 and Unknown 2 are
correct.
CHEM 106L practical Exam.
You will be given five unknown salts. Write down the sample numbers on the vials here.
Samples
Information for these salt samples is given in Table 1.
Molecular
weight
ΔHsol
Na3PO4
119.98 g/mol
-53.67 kJ/mol
Na2CO3
105.99 g/mol
-22.34 kJ/mol
NaC2H3O2
NaC2H3O23H2O
82.03 g/mol
-17.32 kJ/mol
136.08 g/mol
19.66 kJ/mol
2.15
7.2
12.0
6.33
10.33
4.76
4.76
NaSCN
81.07 g/mol
6.29 kJ/mol
1.1
Sample
Sample
number
pKa
Spectrophotometric
information when
mixed with acidic
Fe+3
No color change
Very weak color
change
Very weak color
change
Strong molar
absorptivity at 485
Table 2 Chemical information for the different samples. Use this information to determine how you can identify
each sample.
It is your task to determine which sample is which salt. You will not be given more sample so be
careful with it and use it wisely. After you know which salt is which, write the numbers in the
“Sample number” column in Table 1. The more proof that you have to verify which sample is
which the higher your score will be.
You can use your labs, your book, the internet, anything but your neighbor. Be sure to tell me
which sample is which using the numbers on the samples. Write up an abstract and show your
calculations on the back and then turn it in. You must be finished at the end of the lab period. If
you do a titration the necessary 0.1 M NaOH and 0.1 M HCl will be provided. Also remember
you may not see all the titration equivalence points for polyprotic acids. If you do absorption
work, the Fe+3 solution will be provided.
Results
Results were determined in two basic areas. These areas were evaluated independently
from the grade given on the paper. Dr. Daren Heaton did the assessment evaluation using
comment from Dr. Daniel Scott for future considerations. The students were give a pass or fail
on their notebooks as well as a pass or fail based on their results. One additional area that should
be included in future assessment reports in laboratory technique. This area was left out on this
report because Dr. Heaton did not feel he could evaluate the large number of student taking the
final adequately in this area. Future evaluations will require additional evaluators.
Evaluation the notebook was more subjective than evaluation of results. A pass was on
the results area was given if the student was able to get the correct Ka or the correct concentration
of acetic acid. Notebook evaluation was based on logical flow, the experimental design and
clarity. It is noted that a pass/fail evaluation in this area may prevent the future users of this
information to determine what changes need to be made in teaching correct notebook annotation.
A more detailed assessment tool may need to be developed to determine exactly where students
are lacking in notebook documentation.
 18 of 49 (37%) students were judged to have achieved a pass on the results section of the
basic skills exam.
 19 of 49 (39%) students were judged to have achieved a pass on the notebook section of
the basic skills exam.
Additional Observations
 Many students did not seem to know how to set up the titration apparatus by themselves
although they were able to do it with a lab partner during regular lab time.
 Students taking the exam later in the examination period seemed to perform better than
those taking the exam earlier in the examination period.
 Less than half of the students wore face shield or goggles.
 Many students did not wear close-toed shoes.
 Very few students were able to use unit conversion analysis (dimensional analysis) to
find the concentration of the unknown.
 Some students were helped by the laboratory instructor more than others.
 Very few students took the time to write down a plan for doing the titration in their lab
notes.
Discussion and Future Considerations
It was very difficult for those administering the lab practical to not help students who
were struggling. In some cases this may have affected the results. Perhaps some consideration
should be made to have a more independent evaluator present when the basic skills exam is
given. However, the contribution of “too much” instructor assistance is likely minimal. One
area of large concern with these results is that the exam was given over a period of several
weeks. The students taking the exam later may have been told by students the exact nature of the
exam and prepared specifically for that exam. To be able to evaluate the student’s actual ability,
the exam may need to be given over a shorter period of time. Or perhaps, two distinct exams
could be given.
It is clear that “lab techniques” need to be evaluated. The students were clearly deficient
in many areas of basic laboratory techniques. An outside evaluator (someone not teaching the
class) may need to be present to independently evaluate this area of the exam.
Very few of the students were able to both perform the titration and accurately calculate
the Ka or acetic acid concentration. These findings indicate that we need to look at classroom
and laboratory instruction to determine how we can improve in theses areas. Perhaps a
reasonable initial goal is that 50% of the students are able to achieve acceptable results. This is a
laboratory technique problems as wells as basic knowledge problem as evidenced by calculations
in the laboratory notebook. Clearly, our curriculum must also focus on acceptable notebook
practices. We must also focus on clear safety procedures and appropriate laboratory attire.