General notes regarding lab reports:
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Never use the words “I” “we” “our” “my” “the student” “the experimenter” or
any other word referring to you directly (this includes the use of “one” which
should ALSO never be used). It makes your paper appear unprofessional.
Instead of writing “I added 5.0g NaCl to 50mL water” just write “5.0g NaCl was
added to 50mL water.” Whoever is reading your lab KNOWS you and/or your
partner did it… your name is on the cover page!
Avoid the use of the word “it”- your goal is to be CLEAR and CONCISE.
Samples are "prepared" and graphs are "generated" or "produced" in the labnothing is ever "created."
Although it IS permissible to use the word "weight" instead of "mass," NEITHER
should be used as a verb. Masses may be "taken" or "found" or "discovered", but
chemicals should not be "massed" (used incorrectly more and more frequently)
and absolutely never "weighted."
“Moles” is like “dozen” in that it is a number. Thus, “the number of moles of
chloride” or “the amount of chloride” can be determined, but statements such as
“the moles of chloride” make no sense gramatically or chemically.
No colloquialisms. You can still be funny and lightheated if that’s your style
without being unprofessional.
No contractions. Again, this is formal writing.
Do not attempt to B.S. your way through a lab report...if you don’t understand the
material, seek help!
The confusing misuse of big words is NOT eloquent. You just sound like a dumb
politician trying to sound intelligent.
Expressing yourself so that the reader "kinda gets what you mean" is not
sufficient.
PROOF READ!!! Read your lab OUT LOUD...if it makes no sense, re-write it!
Reading OUT LOUD is a lost art from which a great many of you could benefit.
All data must be expressed with the correct number of significant figures.
This is not the third grade, when you (hopefully) stopped writing “the subject of
my book report is…” so NEVER use the phrase "in this experiment" or any
equivalent thereof ANYWHERE in your report.
Spelling and grammar errors are not acceptable. There are these really cool things
called computers now where all you have to do is press one button and the
spelling and grammar of your document are checked. You cannot possibly be too
lazy to use it. Yes, the computer will find words and phrases that it doesn’t
understand which you wrote correctly. You can simply tell the computer to
ignore those. But it will also find your legitimate errors. Watch especially the
use of “affect/effect,” “it's/its,” “their/there,” “passed/past,” etc. Tests are not
“done” and samples are not “determined.” Whether or not English is your native
language, it IS the internationally accepted language of science. USE the Cooper
Union Writing Center! They’ll proof read your paper for FREE!
Are the words “previously prepared” or “aforementioned” in your report?
Chances are they are unnecessary. Remove them.
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Sample calculations must include 1 complete set of calculations using real data
from the lab. The set of calculations must also include the propagation of error, if
required. Include all units. If the units of the answer don’t match the calculation,
the math can not possibly be correct.
Express numbers in correct scientific notation (3.0x10-6 NOT 3.0E-6). These
computers are amazingly adept at superscripts whereas old typewriters were not.
Justify everything. If you say one method is better than another due to cost, the
dollars involved had better be cited. All error must be explained.
Be specific.
Redundancy is not good. This includes repetition both within and between
sections of your report.
“Within” is correctly used above. Samples are not “within” a solution. Precise
and accurate scientific writing is required.
Redundancy is not good. (annoying, isn't it?!?!)
The states of matter (s), (aq), (g) are NOT subscripted. (You may italicize the
letter if you are so inclined.) The numbers in formulas denoting multiple atoms or
functional groups ARE subscripted. Ex: AgNO3(aq).
Watch Out For Unnecessary Capitalization… This Is Really Not A Good Idea
When Writing A Report Filled With Chemical Symbols. Chemical techniques,
e.g.- mass spectroscopy, may use capital letters for their abbreviations (e.g.- MS),
but are not capitalized when written out. The only exception to this rule is in the
report title. Table captions are not an exception (see any JACS article). Be sure
abbreviations are correct, too: mL, mol, etc.
Every single one of these could be in ALL CAPS, bold, italics. They are all
critical to your development of superior scientific writing skills. Follow them or
suffer the consequences. In cases where there is a discrepancy between this
material and your lab manual (or what your teacher/professor has told you), you
should consider the significance of the difference and who will be reading and
grading your report.
************ BEGIN SAMPLE LAB ************
Determination of the Chloride Content in a
Soluble Salt
By Ion Exchange and Titration
(Don’t just copy the title from the lab manual.)
(Also, note that “Determination of an Unknown Chloride” makes no sense, scientifically
or grammatically. WHAT did you determine about it? Note the following appropriate
definition of the word “determination:” The ascertaining or fixing of the quantity, quality,
position, or character of something: a determination of the ship's longitude; a
determination of the mass of the universe.)
Kevin Kolack, Ph.D.
The Cooper Union
Ch111, Section E
Professor Kolack
April 1, 2010
************ PAGE BREAK ************
Table of Contents
Abstract
3
Introduction
3
Experimental
3
Results and Discussion
5
Acknowledgments (no “e”- see your lab manual)
6
References
6
Appendix I- sample calculations
7
Appendix II- error propagation
************ PAGE BREAK ************
Abstract
The percent chloride of an unknown soluble salt (AxCly, number 169) was determined by
ion exchange to be 31.6% ± 0.5%. The fact that the propagated error was 0.3% indicates
a low incidence of random error. The percent error was 43.5%. The difference between
the experimental and actual percent chloride (55.9%) was due to the use of fictitious data.
Hydroxide ions were eluted when a known quantity of the unknown was passed through a
column of Amberlite IRA-910. This basic solution was over-titrated with potassium
hydrogen phthalate (KHP) and back-titrated with a solution of NaOH prepared in the lab
and standardized with KHP.
PAST TENSE. BRIEF SUMMARY OF PROCEDURE, RESULTS, ERROR, AND
EXPLANATION. RESULTS ARE REPORTED AS ± THE STANDARD
DEVIATION. IF BEING CALCULATED, THE PROPAGATED ERROR IS ALSO
NOTED, SEPARATELY. (THINK OF WHY!)
See your lab manual and a statistics book for a discussion of significant figures,
especially with regard to standard deviation.
Note that percent error for this lab and the gravimetric lab is not just a subtraction of the
actual and experimental (not “determined”) percent chlorides, but is a statistical percent
error.
************ PAGE BREAK ************
Introduction
PRESENT TENSE. BRIEF HISTORY AND TECHNIQUE OVERVIEW, INCLUDING
WHY THE EXPERIMENT IS BEING PERFORMED (OR WOULD BE PERFORMED
IF THIS WERE NOT JUST FOR A GRADE IN A CLASS). So, this lab report should
include: theory of standard solution preparation including the reasons for the
standardization of NaOH and amphoteric nature of KHP; reason for over-titration and
back-titration; questions from manual. MUST INCLUDE REFERENCES TO
CURRENT PERIODICALS, NOT JUST TEXTBOOKS. THIS SECTION IS NOT
JUST A VAGUE RESTATEMENT OF THE EXPERIMENTAL SECTION!!!
Determination of chloride content is important to such varied fields as disposal of
radioactive waste and restoration of classical paintings. One method of finding the
makeup of an unknown is ion exchange…
8
Experimental
(PAST TENSE. WHAT DID YOU DO EXACTLY? SOMEONE SHOULD BE ABLE
TO REPEAT THE EXPERIMENT USING THESE INSTRUCTIONS. THE LAB
MANUAL DOES NOT SAY TO INCLUDE DATA HERE, BUT THIS IS THE
METHOD I CHOOSE TO FOLLOW.)
Preparation of standardized NaOH solution (NOTICE THAT I AM CAPITALIZING
ONLY THE FIRST WORD OF MY TABLE CAPTIONS, SECTIONS AND
SUBSECTIONS)
Approximately 0.1M NaOH was prepared by dissolving 20 pellets of solid NaOH
(approximately 0.1g each) in 500mL distilled water. This solution was standardized by
titrating it three times against known quantities of the monoprotic acid KHP in 50mL
water (Tables 1 and 2). Roughly 0.5g KHP was used in each of the three trials so that the
titrations would be accomplished using approximately 25mL solution. During the
titration of Sample 1, a beaker was broken on a nearby lab bench, and in the resulting
confusion, several drops of NaOH were added to the solution past the end point of the
titration.
Sample 1
Sample 2
Sample 3
Initial mass (g) ± 0.0002g
58.3297
57.8168
57.2967
Final mass (g) ± 0.0002g
57.8168
57.2967
56.7809
Mass of KHP transferred (g) ± 0.0003g
0.5129
0.5201
0.5158
Number of moles of KHP ± 2x10-6
2.512x10-3
2.547x10-3
2.526x10-3
Table 1- Amount of KHP used to standardize the NaOH solution
(NOTE: ALL tables and figures must be numbered and must be referred to in the
text)
Sample 1
Sample 2
Sample 3
Initial volume NaOH (mL) ± 0.05mL
0.00
1.58
2.36
Final volume NaOH (mL) ± 0.05mL
26.89
28.36
29.12
Volume NaOH transferred(mL) ± 0.07mL
26.89
26.78
26.76
Table 2- Volume of NaOH required to neutralize KHP
From the data above, the concentration of the NaOH solution was calculated. This
standardized NaOH was stored in a sealed plastic container until the next lab period.
Preparation of the ion exchange column
A 50mL buret was cleaned until it was free draining. A glass wool plug was inserted into
the buret to prevent clogging of the stopcock by the ion exchange resin. Approximately
35mL of resin was added to the buret in small portions, draining off excess liquid as
needed. The buret was inverted several times in order to ensure even packing of the
column and to remove air bubbles. At no point during the experiment was the liquid
level allowed to drop below the level of the solid resin. The resin was recharged by
passing 40mL of a stock 4% NaOH solution through the buret at a rate of 5mL/min.
Excess NaOH was rinsed from the resin by passing 60mL of water through the column.
(NOTE ON THIS LAB: HOW WERE YOU SURE WHEN THE EXCESS HAD BEEN
RINSED OFF? THOSE INSTRUCTIONS SHOULD BE INCLUDED AS WELL.)
Ion exchange and acidification of unknown
An aqueous solution of the unknown was prepared by adding approximately 0.2g of the
unknown (Table 3) to a 50mL volumetric flask and filling to the mark with water.
Initial mass (g) ± 0.0002g
9.0786
Final mass (g) ± 0.0002g
8.8551
Mass unknown transferred(g) ± 0.0003g
0.2235
Table 3- Mass of unknown used to prepare solution
A volumetric pipet was used to pass exactly 10mL of this solution through the column.
To ensure complete exchange, an additional 80mL of water was passed through the
column. The eluent was collected in an Erlenmeyer flask, and 3 drops of phenolphthalein
were added. A second buret was filled with a stock solution of KHP (0.400M), and the
purple solution was titrated 3mL beyond the equivalence point (when the solution turned
clear). Three drops of CCl4 were added to the acidified solution as a preservative, and the
process was repeated twice more with additional 10mL portions of the solution of the
unknown (Table 4) after first regenerating and rinsing the column as above.
Sample 1
Sample 2
Sample 3
Initial volume KHP (mL) ± 0.05mL
0.08
1.58
2.36
Final volume KHP (mL) ± 0.05mL
5.09
6.73
7.35
Volume KHP transferred(mL) ± 0.07mL
5.01
5.15
4.99
Amount of KHP (moles) ± 3x10-5
2.04x10-3
2.06x10-3
2.00x10-3
Table 4- Volume of KHP used in over-titration of OH--containing eluent
Back titration of acidified solutions
A 50mL buret was filled with the standardized NaOH solution prepared earlier, and the
three acidified solutions were titrated until a faint pink color persisted. The data for these
titrations are given in Table 5.
Sample 1
Sample 2
Sample 3
Initial volume NaOH (mL) ± 0.05mL
15.97
5.98
26.65
Final volume NaOH (mL) ± 0.05mL
22.86
13.19
33.77
Volume NaOH transferred(mL) ± 0.07mL
6.89
7.21
7.12
Table 5- Volume of standardized NaOH used to titrate acidified eluent
Results and Discussion
(WHAT DID YOU DISCOVER?)
Since 2.512x10-3mol, 2.547x10-3mol, and 2.526x10-3mol (± 2x10-6 mol) KHP were
titrated in three runs by 26.89mL, 26.78mL, and 26.76mL (± 0.07mL) NaOH, and each
mole of KHP neutralizes one mole of NaOH, the molarity of the NaOH is calculated to
be 0.09342M, 0.09511M, 0.09439M. However, the relative uncertainty of these numbers
is 0.012, so the molarity can only be calculated to a precision of 0.094M ± 0.001M. This
concentration can then be used to calculate the number of moles of NaOH in the final
back titration (Table 6).
Sample 1
moles NaOH ± 8x10-6
Sample 2
6.48x10-4
6.78x10-4
Sample 3
6.69x10-4
Table 6- Calculation of number of moles of NaOH used in back titration
Because the chloride in the unknown displaces the hydroxide from the resin in a 1:1 ratio
(SHOULD BE EXPLAINED IN INTRODUCTION), the amount of hydroxide in the
eluent of the ion exchange equals the amount of chloride in the unknown. Since acid
(KHP) was added to the eluent past the endpoint, the amount of hydroxide in the eluent is
equal to the amount of KHP added minus the amount of NaOH required to neutralize it in
the back titration.
Sample 1
Sample 2
Sample 3
Number of moles of KHP ± 3x10-5 (from Table 4)
2.04x10-3
2.06x10-3
2.00x10-3
Numbe of moles of NaOH ± 8x10-6 (from Table 6)
6.48x10-4
6.78x10-4
6.69x10-4
Difference (# moles OH- in eluent = # moles Cl- ± 3x10-5)
1.39x10-3
1.38x10-3
1.33x10-3
Table 7- Collection of data used in the determination of moles of chloride
From the number of moles of chloride and the molecular weight of the chlorine atom, the
mass of chloride in the original 0.2235g (± 0.0003g) portion of unknown can be
determined. Since 10mL (± 0.02mL)of the original 50mL (± 0.05mL) prepared was used
for each run, a factor of 5 is necessary in the calculation.
Sample 1
Sample 2
Sample 3
mass of Cl in 10mL of the 50mL/0.2235g sample
0.0493
0.0489
0.0472
% Cl in the sample ± XXXXXXX
110
109
106
Table 8- % chloride in the three runs
The average percent chloride is 108 with a standard deviation of 1.7. There is a high
level of precision, but since each of the three samples contains more than 100% chloride,
an error has clearly been made. The numbers used in this report are fictitious, causing the
error here.
In student labs, the t and Q test could be performed…..these tests could ALSO be
performed on data used earlier in the lab to decide if a sample run should be thrown out
(for instance, in the calculation of the molarity of the standardized NaOH). CONSIDER
THE SIGNIFICANCE (MEANING) OF THE Q, SD and ppt calculations.
The results of this experiment will be compared with that of the gravimetric analysis to
be performed on the same unknown.
SPECIFIC SOURCES OF ERROR MUST BE NOTED, NOT GENERALITIES SUCH
AS "THE BALANCE MAY HAVE BEEN USED INCORRECTLY"… IF THE
BALANCE WAS USED INCORRECTLY, IT SHOULD HAVE BEEN NOTED IN
YOUR NOTEBOOK AND THEN MENTIONED IN THE EXPERIMENTAL
SECTION… THE RESULTS OF THIS ERROR SHOULD BE DISCUSSED IN THE
RESULTS SECTION!!
Acknowledgments (note the correct spelling of the word)
DO NOT thank your professor or the stockroom for doing their jobs, your lab partner for
being your partner, or your roommate for putting up with you and letting you use their
computer….while your gratitude is appreciated, it is unnecessary and unprofessional.
(This, and much of the information presented here, may differ from what is stated in your
lab manual or class. You should consider who is grading your report in deciding how to
write it.) If someone was particularly instrumental in helping you with equations, etc,
they should be mentioned here. If your professor stimulates you into pursuing a graduate
career in chemistry (or discourages it, for that matter!) then (s)he may be mentioned here.
References
Textbooks, the lab manual, AND CURRENT JOURNALS should be consulted. The
"references" section is for items which you "footnote" in the text. WEB PAGES
SHOULD NOT BE USED (with the exception of the online versions of printed
materials) as they are neither static nor peer-reviewed. MANY OF YOU HAVE NEVER
USED ANYTHING OTHER THAN GOOGLE FOR RESEARCH UP TO THIS POINT
IN YOUR EDUCATION… THAT IS VERY SAD. GOOGLE DOES NOT PROVIDE
AN EXHAUSTIVE EDUCATIONAL OR SCIENTIFIC SEARCH. Online research can
be useful in locating reference data, but caveat emptor. An "articles consulted" or
bibliography section may be included for reference materials you used but which you do
not directly reference in the text of your report. The references should be in the
following format, per the American Chemistry Society Style Guide (however, different
journals have different standards- there is no universal agreement on reference format):
book
AuthorLastName, AuthorFirst(andMiddleIfWanted)Initial; OtherAuthorLast,
OtherAuthorFirst. TitleOfBook, 1-WordPublisher: CityofPublication,
YearOfPublication, PageReferenced.
journal
AuthorLastName, AuthorFirstInitial; OtherAuthorLast, OtherAuthorFirst.
TitleOfJournalAbbreviated, YearOfPublication, Volume:IssueNumber, PageReferenced.
examples
Kahn, O. Molecular Magnetism, VCH: NY, 1993.
Wang, S.; Tsai, H.-L.; Hagen, K.S.; Hendrickson, D.N.; Christou, G. J. Am. Chem. Soc.
1994, 116, 8376.
Brown, H.C.; McDaniel, D.H.; Haflinger, O. in Determination of Organic Structures by
Physical Methods; Brand, E.A.; Nachod, F.C., Eds. Academic: NY, 1955, Chapter 14.
Appendix I- sample calculations
(BE ABSOLUTELY SURE SOMEONE LOOKING AT YOUR PAPER CAN
UNDERSTAND AND REPEAT WHAT YOU DID!!!!!)
Preparation of standardized NaOH solution
500mL of 0.1M solution desired
assume each NaOH pellet weighs 0.1g
(0.1mol NaOH/L solution)*(0.5L solution)*(40.01g NaOH/mol NaOH)*(1 pellet
NaOH/0.1g NaOH) = 20 pellets NaOH needed
assume titration with approximately 25mL KHP solution
since KHP is a monoprotic acid, assume 25mL of 0.1M KHP (1 mol acid neutralizes 1
mol base)
(0.1mol KHP/L solution)*(0.05L solution)*(204.22g KHP/mol KHP) = 0.51g KHP
needed
Appendix II- error propagation (not being calculated in Cooper CH111 as
of Spring 2002)
Weighing by difference
Subtracting a final mass (±0.0002) from an initial mass (±0.0002),
so the mass transferred has an uncertainty of
[(0.0002)2 + (0.0002)2].5 = 0.0003
Calculation of # moles of KHP
Dividing a final mass (±0.0003) by an exact molecular weight,
so the moles of KHP has a relative uncertainty (Sample 3) of
[(0.0003/0.5158)2 + 02].5 = 0.0006
So the absolute uncertainty for Sample 3 is (2.526x10-3)(0.0006)=2x10-6
Calculation of molarity of NaOH
Dividing moles (2.526x10-3mol ± 3x10-5 mol KHP = moles NaOH)
by milliliters (26.76mL ± 0.07mL NaOH) (Sample 3),
so the result has a relative uncertainty of
[(3x10-5/2.526x10-3)2 + (0.07/26.76)2].5 = 0.0121611
Thus, the absolute uncertainty for Sample 3 is (0.0943946)(0.0121611) =
1.147947x10-3
Since the absolute uncertainty is reported to a lesser degree of precision
than our answer, the number of significant figures in the answer must be
limited: 0.094 ± 0.001
Send questions or comments to the author via the contact form
© 1999-2010 Dr. Kevin Kolack; may not be copied, reproduced and/or placed on file
without written permission from the author