How to Report Your Laboratory Work

HOW TO REPORT YOUR LABORATORY WORK

You must keep your laboratory work in a bound, quadrille-ruled journal (graph paper pages). On the cover, print in ink your name, Chemistry 2011-2012, and the instructor’s name. Reserve the first page (both sides) for a table of contents, in which you will list the number, title, and pages of each experiment as you do it. Number all pages in ink in the lower outside corner.

Inside the front cover, tape the grading rubric by which I will evaluate your journal. Inside the back cover, tape the laboratory safety rules. On the last few pages of your journal, tape these instructions and information for determining uncertainty in your measurements and calculations.

ON THE DAY BEFORE AN EXPERIMENT prepare your lab journal as follows: Headings: On a new page, write the date of the experiment in the upper right corner. Write and underline the lab title at top center. Purpose: Write a sentence or question that describes the specific goal of the experiment. Procedure: Prepare an outline or flow chart of the procedure. Data: Prepare a table or other suitable format to receive your data. Don't be stingy; leave plenty of space for corrections or unanticipated observations.

The instructor must sign your preparation before you may begin the experiment.

ON THE DAY OF THE LAB WORK record your data immediately in your lab journal in blue or black ink. Never use scratch paper! Include labels, correct significant figures, and units. If you make a mistake, cross it out with a single line and write the correct value next to it. If a problem like a spill spoils an entire section of work, cross it out with a big "X" and continue. Whiteout, erasures, obliterated values, missing pages, or pencil entries are unacceptable.

AFTER THE EXPERIMENT write a complete analysis in your journal. If calculations are needed, show detailed set-ups including units. Round off answers appropriately and include units. Graphs should be done in pencil and must be at least ½ page in size, with an easy-to-use scale, labels, accurate points, and a line showing the trend and consistency of the data points.

After the analysis, write a short discussion in which you report your conclusions –– that is, the answer to the question implied in the purpose –– and address any questions raised in the lab instructions. Evaluate the reliability of your results, focusing your discussion on experimental errors. If some observations were omitted, discuss the reasons here (so I don't think you just forgot to do them).

Grading. In most cases, there will be a quiz on each lab a few days after we finish the experiment (including analysis). The quiz includes an evaluation of your journal, questions about the procedure, questions that test your understanding of the analysis, and questions about the discussion, including error analysis. The quiz is open journal and you hand in the journal with the quiz. Some labs will be graded with journal credit only (no quiz). LAB JOURNAL EVALUATION RUBRIC I will evaluate your journal after each experiment, as part of the lab quiz, using this rubric:

6 Excellent. The story of the experiment is complete and logical, so a reader can easily understand exactly what happened. • experiment is entered in table of contents with page numbers • preparation is complete and signed • data are complete, well organized and clearly recorded with labels, units, and proper sigfigs • analysis is complete and accurate, with all calculation setups shown (including units); graphs are of adequate size, properly scaled, labeled, and accurately drawn • discussion addresses all questions posed in instructions; percent error is calculated (where appropriate) and logical sources of error are suggested.

5 Good. There are a few minor errors, distracting but not enough to interfere with understanding the experiment. Examples of minor errors include • table of contents entry or prep signature is missing • a few (1-2) data items are missing units or proper sigfigs • analysis is complete but poorly organized, or has minor inaccuracies, or does not show all setups adequately, or graph is too small, poorly labeled, or is sloppy, but is not inaccurate • discussion is present but is illogical or incomplete.

4 Fair. The experiment seems to be complete but accumulated errors or omissions interfere with clear understanding. There may be many minor errors (see above), or one or more serious errors. Examples of serious errors include • data are poorly organized, missing several units, have erratic sigfigs, or are poorly labeled, but are complete enough that analysis and discussion are possible • analysis is incomplete (but at least ½ done), or many setups are missing or without units, or calculations are inaccurate enough to be misleading, or graph has several errors such as too small, poorly labeled, scale error, sloppy, or inaccurate, to the point where the results are compromised. • discussion is missing.

3 Poor. Apparently the student did the experiment, but data and/or analysis are too incomplete or unorganized for understanding. For example, • title and/or procedure are missing, so reader must guess what the experiment is about • data are woefully inadequate (unorganized, missing many units, improper sigfigs, poorly labeled) • analysis is substantially (½ or more) incomplete and unorganized • there is no basis for discussion, so it doesn’t matter whether or not it is present.

2 Unacceptable. Perhaps the student did the experiment, but there are so many omissions that very little evidence is recorded in the journal. The title & procedure are missing; data are present but incomplete, unorganized, and poorly labeled; analysis is missing; discussion is missing.

1 Unacceptable. The prep for experiment is in the journal, but it is otherwise blank.

0 Missing. Experiment is not in the journal. LABORATORY SAFETY The chemistry laboratory can be a dangerous place, but it need not be. If you take intelligent precautions, prepare, and learn proper techniques, the laboratory is no more dangerous than any other classroom. Most of the precautions are just common sense.

1. THE LABORATORY IS A SERIOUS WORKPLACE. It is not a place for fooling around. Playful actions that would be harmless in your backyard could be tragic in the lab.

2. YOU MUST WEAR GOGGLES AT ALL TIMES IN THE LAB, even if you yourself are not doing an experiment. Know the location and proper operation of the eyewash fountains.

3. Wear shoes at all times. Tie back long hair or loose clothing, especially near flames.

4. NEVER EAT, DRINK, OR CHEW GUM IN THE LAB.

5. Be prepared. Outline the procedure before coming to lab, and pay special attention to safety precautions. If you arrive unprepared and do not think about what you are doing, you are more likely to have an accident.

6. Work only at your assigned lab station, and do only the experiments assigned by the instructor. Unauthorized experiments are prohibited.

7. Treat all chemicals as hazardous as a matter of habit. Check labels carefully to be sure you have the right bottle. Never return leftover chemical to the stock bottle; discard it.

8. Never taste anything. Never smell anything by inhaling directly over the container; instead, fan some toward you by sweeping your hand over the top of the container.

9. Hot glass looks just like cold glass. Be careful.

10. WASH YOUR HANDS with soap and water before leaving the lab. Try not to touch your eyes, nose, or mouth unless you know your hands are clean.

11. Keep your work area tidy and your equipment clean. Leave unnecessary items like backpacks and jackets in the class area up front.

12. Discard solid chemicals in the waste jar at the front of the room and solutions down the drain, unless otherwise instructed. Never discard any solid material in the sink. If your spill anything, ask the teacher for help with immediate cleanup and disposal.

13. If you break any glassware, notify the instructor and clean it up immediately. DO NOT put it in the trash can; use the special container for broken glass.

14. IMMEDIATELY REPORT ANY ACCIDENT TO THE TEACHER, even a minor one. If you spill anything on your skin or burn yourself, rinse with lots of cold water and notify the teacher. HOW TO DETERMINE THE UNCERTAINTY IN YOUR LABORATORY WORK

RECORD EACH MEASUREMENT CORRECTLY Label the measurement. A measurement means nothing if it is not clear what you measured. Label each measurement! If you are measuring several items, make a table.

Record the unit. A pure number has no units, and can refer to anything; it may even be an imaginary number like the square root of –1. Quantities refer to real things that can be measured using a variety of units. A quantity has no meaning unless the unit is shown.

Record the uncertainty. Because quantities refer to real, measurable properties like mass or volume, every measurement has some uncertainty: at some point you reach the limit of your measuring instrument, and cannot measure accurately beyond that limit. Pure numbers have no such limitation. So a quantity must tell you two things: how large or small it is (its value), plus how well you were able to measure it. The number of significant figures you record gives the second piece of information.

There is no difference between the pure numbers 2 and 2.000, but there is a difference between the quantities 2 g and 2.000 g. The quantity 2 g was measured crudely, maybe just by estimating it, but the quantity 2.000 g was measured carefully, with an accurate balance. The numerical values are the same, but the information about the measurements is not. The quantities contain different numbers of sigfigs.

When reading an instrument, you should record all the digits you can observe up to and including the first digit where uncertainty begins (but no more). For example, a mass reading recorded as 2.560 g shows that the last digit is uncertain, either because you had to begin estimating between the marks on the scale or because the instrument is not designed to be any more accurate. If you wrote fewer digits, you would imply that your measurement was cruder than it really was, and if you wrote more, you would imply that your measurement was more accurate than it really was. You want to communicate exactly what level of uncertainty exists in your measurements.

Here are the units and number of decimal places you can read for measuring instruments you will use frequently in the lab:

Instrument Unit Readability Instrument Unit Readability 3-beam grams 2 decimal places 10 mL mL 2 decimal places balance graduated cylinder 2-place digital grams 2 decimal places 50 or 100 mL mL 1 decimal place balance graduated cylinder 3-place digital grams 3 decimal places buret mL 2 decimal places balance glass °C 1 decimal place digital °C 1 decimal place thermometer thermometer COMBINING UNCERTAINTIES There are two rules for reporting the uncertainty in answers calculated from data: 1. When adding or subtracting, round off to the fewest number of decimal places. Suppose you are adding together these molar masses: 22.9898 1.00794 12.011 47.9982 84.00694 = 84.007 g/mol You would round the answer to 84.007 g/mol, the same number of decimal places as the quantity with the fewest decimal places in your calculation.

2. When multiplying or dividing, round off the final answer to the same number of significant figures as the fewest sigfigs in your data, after any addition or subtraction. Suppose you are calculating this ratio: mass = 35.72 g = 35.72 g = 8.712 g/mL volume 16.5mL – 12.4 mL 4.1 mL After doing the subtraction, you have 4 sigfigs in the numerator and 2 sigfigs in the denominator. You should round your final answer to 2 sigfigs (8.7 g/mL).

The uncertainty in the answer, communicated by your round-off decision, arises not because you are sloppy or unskilled, but because of the limitations of your measuring instruments. You should not report more digits than your measuring instruments can provide.

ZEROES AND SIGFIGS Which zeroes count as sigfigs and which do not? Three simple rules: • Leading zeroes should never be counted as sigfigs. Zeroes at the beginning of the number are never part of the measurement. There are only 3 sigfigs in the quantity 0.00275 kg. • Internal zeroes must be counted as sigfigs. Zeroes in the middle of the overall number are always part of the measurement. The quantity 1.004 g has 4 sigfigs • Trailing zeroes are counted as sigfigs only if the decimal point is written. Zeroes at the end of the number are assumed to be part of the measurement if the decimal point is written, but are assumed to be decimal point locators if no decimal is showing. The quantity 12.40 mL has 4 sigfigs, but the quantity 250 mL has only 2 sigfigs. To be absolutely clear, you can use scientific notation: 2.5 x 102 mL has 2 sigfigs, and 2.50 x 102 mL has 3 sigfigs.

CALCULATING PERCENT ERROR Suppose you reported the density of a sample to be 8.7 g/mL based on your measurements, then found out that the accepted value for the density of that sample is 8.9 g/mL. Your error is

% error = (your answer – accepted value) x 100 = (8.7 – 8.9) x 100 = – 2.2% error accepted value 8.9

The error is negative 2.2%. That just means that your value is 2.2% below the accepted value (if the error were +2.2%, it would mean that your value is 2.2% above the accepted value). Watch the order of operations in the numerator, and do keep the sign in your error analysis.