Educational Objectives: In general, one of the purposes of this lab is to teach you how to use some of the different devices that are used in laboratories. These include beakers, pipets, graduated cylinders, microliter pipets, balances among others. You should be able to correctly use all of the items included in this lab. There is a display of other devices that you will need to know but will not use today. These are other items are listed in your lab manual.
In addition, you should also learn how to record and analyze data. This includes the correct use of SI units (“the metric system”), significant digits, and various statistical tools.
Throughout this semester, you will use the information from this lab so it is important for you to make sure that you know and understand all of this information.
Experimental Objectives: Most labs have an experimental component in addition to the educational one. The aim of these components is to acclimate you to the scientific process and to understand the rationale behind the procedures. This will also allow you to utilize your knowledge of biology, of the techniques used and of data analysis to make a sound conclusion from your data.
In this lab, you will determine which of two instruments is most accurate and precise for delivering a given volume. There are four different experiments (thus you should have 4 different conclusions, one for each experiment) comparing different instruments. This is a slight variation from the procedures in the lab manual, although the particular set up of each experiment is the same. In each case, you will determine the actual volume of water delivered by weighing the water as at room temperature, each milliliter of water weighs 1 gram. This is done as a way to verify that actual volume transferred. Thus, if you measure 60 ml of water in a beaker, it should weigh 60 g. However, there are inherent random errors in using the beaker (or any other device) as a measuring device (such as the 60 ml mark might be incorrect or you may read the volume incorrectly). Therefore, it is unlikely that it will weigh exactly 60 g. To reduce the chance that these errors will skew results, scientists will replicate the experiment many times and average the results. In this manner, it is hoped that the random errors associated with each measurement will cancel each out and the more likely that the average will be close to the actual value. In this experiment, you will perform each experiment 8 times.
- Experiment 1: Which is more accurate and is more precise for delivering 60 ml, a 100 ml beaker or a 100 ml graduated cylinder?
- Experiment 2: Which is more accurate and is more precise for delivering 4 ml, a 10 ml graduated cylinder or a 5 ml glass pipet?
- Experiment 3: Which is more accurate and is more precise for delivering 1 ml, a 2 ml glass pipet or a microliter pipet?
- Experiment 4: Which is more accurate and is more precise for delivering 2 ml, a 2 ml glass pipet or using the microliter pipet twice (1000 ul each time)?
Before coming to lab:
- Read and complete the prelab exercises in the lab manual. While you must do the exercises in the prelab, you will not submit it. It will be obvious to me if you do not prepare for lab and you will find that the lab takes longer – not t mention that it will be more difficult to understand what you are doing..
- Read the lab procedures with the understanding that while the actual techniques (measuring water volume by weighing) is the same, you will only be performing the experiments outlined in the sections above.
- Prepare the Introduction and Procedures section of you lab note book. These areas should contain all the information that you would need to answer any question about the experiments and all information needed to actually do the experiments.
After Lab:
- Analyze your data. For each experiment you should have 8 volumes that you measured for each of the two instruments used in that particular experiment. You need to calculate the average, standard deviation and percent error for each set (as described in excel quiz 2). Be sure that you round all values to the correct decimal place (based on the precision of your measuring devices). ALL data analysis should be in your lab note book.
- For each experiment, perform a t test (as described in excel quiz 2) to determine if the differences can be attributed simply to random errors (and thus you cannot conclude that one device is more accurate) or is it more likely that the differences are a reflection that one device is significantly more accurate than the other. Remember, a p value (what the t test in excel calculates) of greater than 0.05 indicates that the differences are NOT significant and are due to random errors, thus each device is equally accurate; a p value equal or less than 0.05 indicates that the values are significantly different and you can conclude that one instrument is more accurate. Helpful hint: set the number of decimal places for the p value to 2 so that you can easily see if it is larger or smaller than 0.05.
- You do not need to prepare a typed lab report, although you must analyze your data and included your results, statistics and conclusions in your lab notebook. It is expected that you will have already analyzed your data prior to taking the Post lab quiz.
Sample data analysis: This is not meant to be a template for your lab report, just an example of how to analyze and interpret your data.
Assume the following data were recorded for an experiment in which one used similar procedures as in your lab but were determining the more effective device for measuring 40 ml, a 50 ml beaker and a 50 ml Erlenmeyer flask.
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Table 1: Beaker vs. Flask |
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|
Beaker (ml) |
Flask (ml) |
|
|
39.4 |
37.6 |
|
|
38.2 |
38.3 |
|
|
40.2 |
38.9 |
|
|
39.3 |
37.8 |
|
|
38.6 |
37.9 |
|
avg |
39.1 |
38.1 |
|
st dev |
0.8 |
0.5 |
|
% error |
2.15 |
4.75 |
|
t test: p = |
0.04 |
|
For this experiment, the average volume delivered by the beaker was 39.1 ± 0.8 ml while the flask delivered 38.1 ± 0.5 ml. These data were found to be significantly different as p ≤ 0.05 therefore we can conclude that the beaker was significantly more accurate (a smaller % error) than the flask.
Real Data:
The following data were obtained by measuring the cross-sectional area of each axon (the tiny circles in the following figure) in the fourth lumbar spinal nerve of normal (NTG) and transgenic (TG) mice. The transgenic mice contain a human gene that leads to neurodegeneration. The analyzed data (average, standard deviation and p value from the t test) are shown below the figures. Based on this data, is the average axonal area of the NTG larger than the TG or are the differences simply due to random errors?