Writing Conclusions

Writing Conclusions
 We used to have students write entire
formal lab reports.
Date of experiment
Title of experiment
Chemical equation (if applicable)
Results and Conclusions
Your signature and date
 We quickly discovered that the
problem areas were
 Purpose
 Results and Conclusions
 In this course, the focus will be
on Results and Conclusions
 A restatement of the purpose is included in
the conclusion.
Remember This
 When you write your conclusions,
write them as if they are at the
end of a formal lab report.
 So, you have already discussed reagents,
procedure, and calculations.
 In the conclusions, you DO reiterate the
 In the conclusions, you only repeat
important data and show how the data
impacts your success at achieving the
The Conclusion Starts with a
Statement of Purpose
 Literally.
 “The purpose of this experiment was to
separate an unknown acid (#8B) from a
mixture using acid base extraction, purify it
by recrystallization from methanol, then
measure its melting point and use it to
confirm the identity of the acid from a list
of possible unknowns.”
 If you feel this is too simplistic a writing
style, you are welcome to make it more
sophisticated…but not at the expense of
covering the facts!
The Purpose is Different with
Virtually Every Experiment
 As such, this means thought must be
given as to why you are doing this
particular lab.
 A clear statement of purpose makes writing
the rest of the conclusion much easier as
you then show whether you achieved the
purpose and show supporting data or
whether you failed to achieve the
purpose…again showing supporting data.
 The statement may be short and simple, or
it may require several sentences.
Often, your conclusion
should include
 a physical description of the
substance you are working with
 solid or liquid, color (clear is not a
color), odor if distinctive (do NOT
intentionally smell organic
 relevant data
 bp, mp, solubility, RI, IR, NMR, MS
are possibilities
 These will depend on the purpose.
If you are identifying an unknown
from a list of possible compounds
 Use one piece of physical data to
narrow the possible compounds
to a “short list,” being careful to
state explicitly your criterion.
 Often this is mp or bp, but may be
solubility, refractive index, IR
absorptions, etc.
Explain How You Created the
“Short List”
 Explicitly state the criterion you
used, being realistic.
 “I measured the mp of the unknown as
120-122°C. I chose all compounds
within 5% of that value (compounds
with mp’s in the range 114 - 128°C).”
 If there are 10 compounds which meet
this criterion, so be it and report each
one with its mp.
Explain How You Created the
“Short List”
 Understand that all compounds in
the short list are equal!
 114°C is not better than 120°C. Try not
to have too high an opinion of your
Identifying your unknown
from a second data point
 For instance, refractive index.
 State the new criterion, then apply
it only to the compounds in the
short list, listing RI values for
every compound in the short list,
and explaining how the compounds
fit / don’t fit the criterion.
 At this point, you may have
identified your unknown.
Use remaining data points
to confirm your choice
 If you have, say, an IR spectrum,
of the compound, use it to
confirm your identification.
 If it cannot be used to confirm, it is
likely you picked the wrong
Dealing with IR Spectra
 First, identify the major peaks
(not all of which will be large)
 the alcohol or acid O-H str
 the sp2 and sp3 C-H str
 the C=O str and state aldehyde,
ketone, etc and whether it is
 the C=C for isolated double bonds and
for aromatic compounds
 all major peaks above 1500 cm-1
Dealing with IR Spectra
 Based on your peak identification
- which is of bonds! - state what
functional groups must be
 It may also be necessary to state
what functional groups are NOT
present (in particular, the
absence of C=O eliminates a lot of
Dealing with IR Spectra
 Your other data MUST be consistent
with your IR data!
 If your bp data says your compound
can only be an aromatic amide and
your IR spectrum has an acid O-H
str, which is right? What do you
 Be prepared to consider the
possibility that you may have
performed a technique incorrectly.

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