Conductivity Lab Review
Configuration of the Lab Station
The left side image shows the entire station including the reagents,
the burette for water and the computer required for the data
acquisition and analysis. The right side image provides a closer
look at the required reagents and hardware.
Conductivity of NaCl Solution
After we delivered 25 ml of distilled water to the beaker
we then tested for the conductivity of the control (only
water). After the data was recorded by the computer we
then delivered 5 drops of the NaCl solution to the test
sample.
NaCl Test Continued
The above images show from left to right the conductivity
of the test solution after 5 drops, 10 drops and 30 drops of
the NaCl reagent were added respectively. The linearity of
the data confirms the increase in conductivity as the
concentration of ions increases.
Molecular Potential
Before we tested NaCl we speculated on the
expected performance. The molecular profile for this
substance is one of an ionic compound. When NaCl
(which is a salt) is dissolved in water we expect the
“liberation” of the ions by what we like to think of as
our water taxi.
NaCl(S) + H-O-H
Na1+(aq)
+ Cl1-(aq)
Since conductivity requires the movement of charge and
these liberated ions are now free to move the increase in
conductivity confirmed our prediction.
Conductivity of CaCl2
Before we test CaCl2 we will again speculate on
the expected performance. The molecular profile
for this substance is again ionic. When CaCl2
(which is again a salt) is dissolved in water we
expect the “liberation” of the ions by the water taxi.
CaCl2 (S) + H-O-H
Ca2+(aq)
+ 2 Cl1-(aq)
Since conductivity requires the movement of charge
the reaction should liberate the captive ions an increase in
conductivity is expected.
CaCl2 Data
From the image of the data it is clear that CaCl2 is
conductive. The slope of the graph shows the solution to
be more conductive than the similarly profiled NaCl.
CaCl2 Data Continued
After the final 5 drops were added the appearance of the graphs
suggested further analysis. Both of the graphs were linear with
fantastic C.O.R. values. The question that plagued us was why the
two ionic compounds were not identical. They were both “binary”
ionic compounds and both “liberated” metallic ions and chloride ions.
It’s a Question of Number
When we considered the reactions we noted that the number of ions
liberated by the salts was not the same. A ratio of 3:2 is observed for
the CaCl2 salt compared to the NaCl salt. When we compared the
slopes of the graphs we obtained a value of 1.5. Since the conductivity
is generated by the movement of ions and the ions are “liberated” in a
ratio of 3:2 the slope of 1.5 strongly supports the model.
Molecular Profile of HCl
To assist in generating a molecular profile for a
substance we generally consult ASIMS. Hydrogen
presents a half filled energy level so it is not
expected to form ionic compounds. We expect this
binary compound (HCl) to be molecular with bonds
that are polar covalent. The reagent list noted the
HCl system as hydrochloric acid. The behavior of
this system may well help us to understand the
important components of the acid profile.
HCl Results
The 5 drops of HCl had a very steep slope. By the
time 30 drops were added the relationship was
clearly established.
NaOH Results
The NaOH graph is the lower of the two presented in the image.
Although both were made at the same time and at the same
concentration the “years” of storage has caused the NaOH to become
less basic. Over time bases absorb CO2 (which forms carbonic acid).
What is important to note is that the ionic compound NaOH is
conductive as expected.
The Magic of Titration
When Mr. Zachmann asked the class to speculate on the
conductivity of a system where a conductor like HCl is added to
another conductor like NaOH. Many suggested an increase in
conductivity would occur. The data however, does not confirm this
hypothesis. During the titration of the basic solution we noted a
decrease in the conductivity. This observation helps us to
understand at the molecular level the act of “neutralization” when
acids and bases interact.
Neutralization
As we consider the act of neutralization we see the
reformation of water by the liberated H+ ion and OH1- ions.
The remaining Na1+ and Cl1- ions continue to support
conductivity.
NaOH(S) + H-O-H
Na1+(aq) + OH1-(aq)
HCl (aq)
+ H-O-H
H1+(aq) + Cl1-(aq)
_________________________________________________
NaOH(S) +
HCl (aq)
Na1+(aq) + Cl1-(aq)
Titration Review
The beaker shows the
titration 1 drop before
equivalence. Notice the
slight pink color. The graph
shows the same system
one drop past equivalence
(the pink color left the
system). Notice what
happens to the graph
when more titrant is added
(see next slide)
Titration Review Continued
The left image was taken at five drops past end point and
the right image was for 10 drops past end point.
Remember that indicators rely on color to convey data and
this can be ambiguous. In the next slide we used an
algebraic approach to determine the equivalence point for
the titration.
Algebraic Approach to Equivalence
The ambiguity of the colorimetric determination of
equivalence can be eliminated by finding the common
(x, y) coordinate for the intersection of the two lines.
The Algebraic Solution
For the declining line y = -19.6 x + 742 and for the ascending line y = 55.4
x -749.5.
If we equate the two respective expressions for the common y value we
get:
-19.6 x + 742 = 55.4 x – 749.5.
Collecting terms we get:
75 x = 1491.5
Solving for x we get:
X = 19.89 drops
Can we use this information to compute the concentration of the base? If
you remember from our solutions unit VM = VM. Therefore the molarity
of the base would be:
M(Base) =(VAcidMAcid)/ VBase = (19.89 drops)(.10M)/(30.0 drops) = .066 M
Applied Research
Using the same technique we measured
the conductivity of solutions made from
some common chemicals found in the
home. Through the exercise we tried to use
ASIMS or deductive process to make
predictions. The data provided in the
following slides will often retain the record of
previously tested materials. This is done to
remind you of the deductive process.
Applications
The substances which appear in
the image to the right were
assessed for conductivity. Methanol
solution was tested first. This test
was followed by ethanol solution,
ammonia solution, sugar in solution
and finally baking soda (AKA
sodium bicarbonate). Each
substance was also tested with
phenolphthalein. As the testing
progressed deductive and inductive
process was used to help with the
predictive process.
Methanol Assessment
The top image shows that
the methanol was not
conductive. The bottom
image indicates that the
phenolphthalein did not
have a positive reaction.
We conclude from this
result that the OH attached
to the carbon of methanol
was not liberated by the
water solvent.
Ethanol Assessment
Because ethanol was
structured very much like
methanol we expected
the same behavior. From
the images to the right
we can see that there
was no difference in their
respective behavior. We
conclude that alcohols
are not conductive.
Ammonia Assessment
The structure of ammonia
is interesting as it presents
a nonbonding electron
pair. The conductivity test
was positive as was the
phenolphthalein test. We
conclude ammonia was
able to attack water or
water attacked ammonia
and as a result liberated
the OH1- ion.
Sugar Assessment
Like methanol and ethanol
solution the sugar did not
conduct nor did it present a
positive phenolphthalein
response. The structural
formula for the glucose showed
numerous OH groups attached
to the carbon backbone. The
tests indicate that these groups
stay attached.
Sodium Bicarbonate Assessment
The photo to the right
shows the response to the
addition of the sodium
bicarbonate. The early
spike in conductivity is the
sodium bicarbonate. The
later response was the
ammonia response that
was not removed so that
we could compare the
magnitude of the
response. Again, we had a
positive phenolphthalein
response.
Lab Report Expectations
In class we outlined the talking points that must appear in the
report. I expect to see several paragraphs dedicated to these various
talking points. I expect a drawing with suitable development to support
your paragraph on the nature of conductivity. As you discuss the
molecular potential of the various tests I expect you to write the
reactions and make clear supporting comments that illuminate the
components of the written reaction which support conductivity (or do
not support conductivity). The titration deserves a separate paragraph
in the introduction. Be sure to explain the titration process and our
expectations with respect to the conductivity of the system as the
titration progresses. Again you will need to write chemical equations to
support your writing.
The discussion part of the report should have equal attention. Again,
I expect well constructed paragraphs. I certainly expect to see
downloaded images with supporting statements. This is particularly
important for the titration part of the lab.
The highly motivated student will devote a separate paragraph to
each of the assessment substances. Again I expect to see downloaded
results followed by a discussion of the formula that I provided on the
board and how this formula interacted to provide or not provide
conductivity.