General notes regarding lab reports:
- 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 lab- nothing 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."
- “Mole(s)” is like “dozen(s)” 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 grammatically or chemically.
- Tests are not “done” and samples are not “determined.” They are "spectra," not "graphs." Glassware is "cleaned," not "cleansed."
- 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. Whether or not English is your native language, it IS the internationally accepted language of science. USE your school's writing center! They’ll proof read your paper for FREE!
- Verbs should not be used as adjectives in an effort to economize your writing. It just makes for awkward writing. For example, “the samples were obtained” is correct, while “the obtained samples” is incorrect.
- Are the words “previously prepared” or “aforementioned” in your report? Chances are they are unnecessary. Remove them.
- Many of the things you use in the lab are precision measuring devices. Thus they are "instruments" not "machines." Do not use the latter word. That being said, YOU are operating the instrument, thus "an IR spectrum was acquired using a SmithsIR" is correct, while "the graph was measured by a SmithsIR machine" is incorrect for no less than 3 reasons. You can quantify something "via" a technique, but using "by" the technique makes it sound like the technique did it all by itself.
- 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 with superscripts (3.0x10-6 NOT 3.0E-6 or 3.0x10^-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 (this is the one and only difference between these instructions and The ACS Style Guide- I choose to make this rule because inexperienced writers have proven incapable of deciding what words are important, and unacceptable inconsistencies generally result). 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, 2011
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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 | 8 |
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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. If this were a synthetic organic chemistry paper, the reaction scheme would go here.
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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!!! If this were a synthetic organic chemistry paper, the reaction mechanism would go here.
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…
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 (± 2x10-6mol) 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 | Sample 2 | Sample 3 | |
| Amount of NaOH (moles) ± 8x10-6 | 6.48x10-4 | 6.78x10-4 | 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 (Table 7).
| 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 number 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. This data fictitious, resulting in 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 percent chloride in an unknown was successfully determined via an ion exchange method, and the results 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!! AND DON'T FORGET A CONCLUDING THOUGHT TO THE STORY THAT IS YOUR REPORT.
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.
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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 The ACS Style Guide
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.
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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
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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