General Experimental Guidelines

GENERAL EXPERIMENTAL GUIDELINES A) General Experimental Guidelines The laboratory is a critical component of your study of chemistry. Therefore, a student must complete all of the assigned laboratory work, including all on- & off-line postlaboratory exercises, in order to pass this course. A-1. Pre-Laboratory Preparation Many of the Chemistry 2 laboratory experiments are intricate and use chemicals that could present a hazard if used improperly. Thus, students are required to judiciously prepare for each experiment by carefully reading the experiment and writing a Title, Purpose, Procedure (brief outline), and Data (outline) section before arriving at the laboratory. A detailed description of each section is described below under, "Writing a Laboratory Report". After preparing the laboratory notebook, students will complete the on-line prelaboratory presentation and must pass the pre-laboratory quiz. Any student without this preparation completed at the beginning of the laboratory period is deemed unsafe and must leave the laboratory until the pre-laboratory write up is complete and the supervising TA is convinced that you are prepared to begin the experiment. A-2. Data Collection All data must be recorded in ink directly into your laboratory notebook. At the completion of the experiment, you must turn in a copy of your data sheet to your TA before you leave the laboratory. A-3. Unknowns Students will obtain all unknowns from the TA. Students must be explicit in their request for an unknown; that is, they must know the name of the experiment and unknown. If a student needs more unknown, they should notify the TA who will then write a note of explanation that the student can take to the dispensary. The note should contain the student's name, the student's locker number, the laboratory section number, the TA's name, the experiment name, and the name of the unknown. A-4. Writing A Laboratory Report Below is the suggested format that your report should follow. Portions of the report should be written in your laboratory notebook and others will be submitted on-line as part of the post laboratory exercises. Post laboratory exercises are due one week after the completion of the laboratory. Below is a general outline of a common format that is often used in science laboratory courses. Discuss this format with your TA during the first laboratory period so that you clearly understand what will be expected. Title: The report should have a title that concisely describes the experiment. Purpose: A brief and concise statement that describes the goals of the experiment and the methods employed. Any pertinent chemical reactions are generally indicated. State A-1 GENERAL EXPERIMENTAL GUIDELINES the purpose of the experiment in the form of a complete sentence. Do not start with the word "To." Procedure: A brief and concise outline of each step of the experiment should be included. If you are using a published procedure, you should also cite the literature or laboratory manual. A drawing of the apparatus may also be included. Data and Observations: Report all measurements and observations that are pertinent to the experiment. Be sure to note any problems or unexpected occurrences. It is important that this section be as neat and as organized as possible. The use of tables will often help in this regard. All data must be recorded in ink directly into the notebook at the time it is collected. A severe penalty will be imposed for pencil or transcribed data entries. Do not erase mistakes. Simply draw a line through the error and record the correction. Your notebook is subject to examination at any time. The following sections are to be submitted on-line as part of the post-laboratory exercise: Calculations: This section generally includes any complicated calculations that are involved in the experiment. Again, it is important to use foresight when organizing this section. Questions: All assigned questions are answered in this section. Results & Conclusions: Report the outcome of the experiment. All reports must be written in non-erasable ink. A date should be indicated on each report, especially in the Data section. You must prepare for each experiment by writing the Title, Purpose, and Procedure before coming to the laboratory. If it is not completed the student must finish it and the TA will deduct 30% of earned points from the post-laboratory exercise for first time offenders (70% for repeat offenders). Students will not be granted extra time to complete the laboratory. It is also important to organize and prepare the format of the Data section before coming to the laboratory so that you will only need to neatly record your data and observations during the experiment. Each section should be clearly marked with a proper heading. Your notebook should be organized and written in such a manner that another chemist could read it and repeat the experiment in precisely the same way. It is also important to complete the report as soon as possible after the completion of the experiment as this is much more efficient than waiting until the night before the experiment is due. A-2 GENERAL EXPERIMENTAL GUIDELINES A-5. Statistical Treatment of Data Every measurement made in the laboratory is subject to error. Although you should try to minimize error, two types of errors will occur. Systematic or Determinate Errors are those errors which are reproducible and which can be corrected. Examples are errors due to a mis-calibrated piece of glassware or a balance that consistently weighs light. Random or Indeterminate Errors are due to limitations of measurement that are beyond the experimenter's control. These errors cannot be eliminated and lead to both positive and negative fluctuations in successive measurements. Examples are a difference in readings by different observers, or the fluctuations in equipment due to electrical noise. You will be graded by your ability to obtain accurate results. Accuracy describes how close your result is to the true value. Another related term is precision. Precision describes how close your results from different trials are to each other. Data of high precision indicates small random errors and leads experimenters to have confidence in their results. Data that is highly accurate suggests that there is little systematic error. A well-designed experiment (and a well-trained experimenter) should yield data that is both precise and accurate. In an effort to describe and quantify the random errors which will occur during the course of the Chemistry 2 laboratory you will be asked to report an average, a standard deviation, a 90% confidence limit, and a relative deviation. You may also have to analyze multiple trials to decide whether or not a certain piece of data should be discarded. The following sections describe these procedures. Average and Standard Deviation The average or mean, x, is defined by x = xi N where each xi is one measurement and N is the number of trials or samples. The standard deviation, , measures how close values are clustered about the mean. The standard deviation for small samples is defined by  =  (xi - x)2 N-1 The smaller the value of  the more closely packed the data is about the mean, or, in other words, the measurements are more precise. Confidence Limits Confidence limits provide an indication of data precision. For example, a 90% confidence limit of 2.0 indicates that there is a 90% probability that the true average of an infinite collection of data is within 2.0 of the calculated average of a limited collection. Clearly A-3 GENERAL EXPERIMENTAL GUIDELINES the more precise a set of data, the smaller the confidence interval. Thus, a small confidence interval is always the goal of any experiment. In General Chemistry you will be required to calculate the 90% confidence interval for all experimental collections of data. The formula to do this is: Confidence Limit = t N where t varies with the number of observations. For the 90% confidence limits you are asked to calculate: Number of Trials (N) 2 3 4 5 6 t 6.314 2.92 2.353 2.132 2.015 You should always report your result as the average ± the 90% confidence limit. Relative Deviation The relative average deviation, d, like the standard deviation, is useful to determine how data are clustered about a mean. The advantage of a relative deviation is that it incorporates the relative numerical magnitude of the average. The relative average deviation, d, is calculated in the following way. a) Calculate the average, x, with all data that are of high quality. b) Calculate the deviation, |xi - x|, of each good piece of data. c) Calculate the average of these deviations. d) Divide that average of the deviations by the mean of the good data. This number is generally expressed as parts per thousand (ppt). You can do this by simply multiplying by 1000. Please report the relative average deviation (ppt) in addition to the standard deviation in all experiments. Analysis of Poor Data: Q-test Sometimes a single piece of data is inconsistent with other data. You need a method to determine, or test, if the data in question is so poor that it should be excluded from your calculations. Many tests have been developed for this purpose. One of the most common A-4