Lab 2 - Scientific Measurement

Transcription

1 Name: Lab 2 - Scientific Measurement As a biology student you will often be asked to make observations. These observations will be of either a qualitative or quantitative nature. A qualitative observation is a descriptive representation of a characteristic and is not numerical. Quantitative observations are numerical. Although qualitative observations can be valuable, quantitative observations are easier to evaluate and are preferred in laboratory science. For example, if you were studying rainfall in Florida and wanted to determine which city had the most rainfall last year, you could describe qualitatively (lots of rain, a little rain or tons of rain), but a quantitative description, e.g. 52 inches, would be much more useful. The metric system is universally used in science. The next few activities are designed to familiarize you with the metric system and its prefixes. The metric system reference units are the meter (m) for length, the liter (l) for volume and the gram (g) for mass. Prefixes are used as part of a unit to indicate a portion or a multiple of the reference unit. For example, the prefix milli (m) indicates (1/1000). A millimeter (mm) is (1/1000) of a meter. A milliliter (ml) is (1/1000) of a liter. A milligram (mg) is (1/1000) of a gram. Note that the same prefixes are used throughout the metric system. The table below lists the most common prefixes that you will come across in this course. Table 1. Common prefixes of the metric system Prefix Abbreviation Relationship to Reference Multiply by Kilo k One thousand times 1,000 Centi c One hundreth 0.01 Milli m One thousandth Micro μ One millionth Nano N One one-hundred millionth

2 Measurement tools There are a number of tools that you will use to measure volumes of liquids in this class. A graduated cylinder is the best way to measure volumes from 5mL to around 1000mL. For smaller volumes (0.1mL to 5mL) you will use pipettes. Although beakers and flasks will often contain graduations (volume markings), they are the least precise tools for measuring and are primarily used for mixing and containing liquids that you have measured with another tool. Beakers have a large mouth and are useful for containing liquids and boiling liquids. Flasks have a narrow mouth and are useful for containing liquids with fumes or that could be harmful if spilled or splashed. They are also useful for swirling mixtures as they are they prevent spilling. Experiment: Making Accurate and Precise measurements. Many of the experiments you will do this semester involve making both accurate and precise measurements. There is a big difference between accuracy and precision. Accuracy is a measurement of how close to the actual value your measurement falls. Precision is a measure of how close your measurement falls relative to your other measurements. In some experiments it is important to be accurate (to put the desired amount of sample into the reaction). In other experiments it is more important to be precise (to do the experiment the same way in each trial). This first exercise is designed to give you some insight into this process. You will be making two kinds of measurements involving liquids (volume) and weight (mass). Graduated Cylinder Erlenmeyer Flask Beaker Transfer Pipette 10mL Pipette 15

3 Materials: 10 ml pipette 1 ml pipette 1 ml graduated transfer pipette Adjustable pipette Pipette tips 250 ml beaker 50 ml beaker Pure water Two decimal place laboratory balance Procedure: Add about 200 ml of pure water to a clean 250 ml beaker. Tare an empty, clean 50 ml beaker. Next with a disposable 1 ml pipette add 5 ml of pure water to the beaker and weight the beaker on the same balance and record the weight on table 1. Repeat this procedure 9 more times recording the weight each time. Next, pipette 5ml of water 10 times using the other measuring tools available (10 ml glass pipette, the 10ml graduated cylinder and the adjustable pipette set at either 500 ul or 1ml) and record all of the data in table 1 on your lab worksheet. Mean - accuracy of a measurement system is the degree of closeness of measurements of a quantity to that quantity's actual (true) value. Standard Deviation - The precision of a measurement system, also called reproducibility or repeatability, is the degree to which repeated measurements under unchanged conditions show the same results. The smaller this number is, the less variation there was between trials. To calculate Mean, sum all of your measurements and divide by the number of measurements. The calculation for standard deviation is a bit more complicated. You need to calculate the square root of the average of the squared differences from the mean. 16

4 1. 5 km = m mL = L 3. 10g = mg cm = m 5. 4 kg = g hm = m kg = mg cm = dm Table 1. Results from pipetting practice L = ml km = dm cm = m g = mg mm = m mm = cm mm = dm 16. 1L = ml Pipette 10 ml 1ml 1ml transfer Adjustable ml Replicates g H g H g H g H Mean STD

5 17. How accurate was your pipetting (look at the mean)? 18. How precise was your pipetting (look at the STD)? 19. Which pipette had the greatest accuracy and which had the greatest precision? 20. Why is accurate/precise pipetting important for biology lab? 20