Real World Metric System

Transcription

1 Real World Metric System Learning Objectives 1. Identify the metric units of weight, length, volume and temperature. 2. Make measurements using the metric system. 3. Convert values between different metric units (ex. grams to milligrams). 4. Convert values between English system units and metric units. 5. Identify the names and functions of laboratory equipment used to measure weight, length, volume, and temperature. 6. Make measurements using laboratory equipment (i.e., scale, ruler, graduated cylinder, and micropipette). 7. Use and identify scientific notation. Introduction Have you ever looked at a Nutrition Facts sheet from a fast food restaurant? See Figure 1 as an example. Did you notice that the serving sizes and quantities of fats, sugars, etc. are listed using grams (g) or milligrams (mg)? These are units of measurement in the International System of Units (SI), commonly known as the metric system. The metric system is the only system of measurement used in most countries worldwide and in science. In the United States, we still tend to use the English system, which includes ounces and pounds. All measurements throughout your scientific career should be stated in metric system units. During lab, we will often take measurements of three properties: length, volume, and mass. The base metric unit for length is meter (m), the base metric unit for volume is liter (l), and the base metric unit for mass is grams (g). Within each category, the metric system makes use of prefixes to change the value of the unit by powers of 10 (Table 1). This makes conversion from one unit to another very easy within the metric system. Remember to always include your units when taking measurements in lab. Figure 1. Nutritional facts for a grilled chicken cool wrap from Chick-Fil-A. 1

2 Table 1. List of common prefixes of the metric system, their symbol, relative value, and scientific notation. Conversion between related metric units is accomplished by moving the decimal point the appropriate number of places left or right (Figure 2). Figure 2. Metric unit conversion staircase. Move up the staircase to larger units, down to smaller ones. Example: to convert 2812 micrometers (μm) to millimeters (mm), move the decimal point 3 places to the left (2.812 mm). 2

3 Conversions can also be accomplished by writing the conversion as an equation, example calculations are shown in Figure 3. These calculations can be used to convert between related units within the metric system but also to convert between the metric and English systems. To convert you will need to know the conversion factors. You will be provided with metric to English unit conversion factors but you must learn the metric unit prefixes and be able to use these in conversion calculations. You may have noticed that Table 1 also lists relative values in scientific notation. Scientific notation is commonly used in science as a type of short-hand when dealing with very small or very large numbers. Notice that the exponent number is the number of decimal places either after (for large numbers a positive exponent) or before (for small numbers a negative exponent) the base unit. It is conventional to express numbers in scientific notation with just one number to the left of the decimal point. Example: 3,756,000 can be expressed as x 10 6 and can be expressed as 8.5 x 10-8 Activity 1: Mass Mass, also referred to as weight, describes how heavy an item is. In the laboratory we often use an electronic scale to measure the mass of an item. Do not forget to zero out your scales when necessary. Use the provided electronic scale to measure the weight of each of these items. Be sure to indicate the unit of measurement. Weight of your smart phone: Weight of your pencil: How does this compare to the weights you estimated in your pre-lab activity? 3

4 The information provided in Table 2 below is from a Nutrition Fact sheet for a popular fast food restaurant. Determine the total amount of fat, salt, and sugar in the entire meal (in both metric units and English equivalents; 1 gram = ounces). Table 2: Fat, salt and sugar content for a meal at a fast food restaurant. Determine to total amounts for the entire meal in metric units and convert to English equivalents. Meal Components: Fat (g) Sodium (mg) Sugars (g) Big Mac Large Fries Large Sweet Tea McFlurry with M&M s Total (metric units) Total (English equivalents) Use the electronic scale to measure out 1 gram of fat, salt, or sugar (you only need to select ONE to measure) using Crisco, table salt, or granulated sugar provided by the instructor. Now use the scale to measure out the total fat, salt, or sugar in the meal from table 1. What is your impression of the quantity you measured? Were you surprised or did you already have some concept of these masses? Activity 2: Length Length describes the distance between two points. In the laboratory we often use a ruler to measure the length of an item (Figure 4). It is important to remember that most rulers we use in lab have units for both the metric and English system. Make sure you use the metric side unless otherwise instructed by your instructor. Use the provided ruler on your benchtop to measure the diameter of a penny. Length in inches: Length in cm: Length in mm: Most students know their height in feet and inches (e.g. 5 foot 6 inches). If you do not know your height you can measure it using the measuring sticks located at the front of the room. What is your height? What is your height in inches only (1 foot = 12 inches)? What is your height in meters (1 meter = inches)? 4

5 Activity 3: Volume Volume is the space occupied by an object. In the laboratory we often use a graduated cylinder to measure the volume of a liquid. If the amount of liquid is very small, we may use a micropipette to measure the volume. Part 3A: Graduated Cylinders There are several sizes of graduated cylinders used in a lab (e.g., 10 ml, 100 ml, 1000 ml). It is important to use the appropriate size when measuring specific amounts of fluid. Additionally, when measuring water and most other liquids in a graduated cylinder a meniscus can form. A meniscus is the curve seen at the top of the liquid in response to surface tension of the liquid and adhesion to the wall of the container. Use the bottom of the meniscus to determine the amount of fluid in the graduated cylinder (Figure 5). Choose an appropriate graduated cylinder from the instructor s bench to measure out the volume of a 1.93 ounce bottle of 5-hour Energy in ml using tap water (use calculations from your pre-lab activity). What size graduated cylinder did you use? Was a meniscus present? Figure 5. Graduated cylinder with meniscus. Part 3B: Micropipettes In the biology laboratory, we often work with very small volumes and it is important to understand how to dispense these small volumes consistently for accurate results in experiments. Micropipettes (also known as mechanical pipettes) are used to measure and dispense small volumes of liquid. You will be required to use a micropipette several times throughout the semester, so it is critical to take the time to learn proper pipetting techniques and practice with each pipettor today. A micropipette allows volumes in the microliter (μl) range to be measured. 1μl = 0.001ml. There are three different micropipettes used in our lab varying in the range of volumes they can transfer. Pipettors from different manufacturers may differ a little bit but the basic understanding remains the same. A general overview of the parts of the pipettes and different size pipettors is described in Figure 6. 5

6 Determine which pipettor you would use to dispense the following volumes: 10µl 0.05 ml 30µl 0.1ml 100µl 0.25ml 600µl 1ml The volume set to be dispensed can be adjusted by rotating the volume adjustment knob that is below the plunger (refer to figure 5). This knob has + and signs on it to indicate increasing (+) and decreasing (-) volume. NOTE: DO NOT SET ANY PIPETTE OUTSIDE OF THE MENTIONED RANGE (E.g.: Never set a P1000 above 1000 or below 100μl). This will put the pipette out of calibration. The yellow line on the P-20 and P-200 pipettes in our lab indicate decimal points. Please note that yellow lines in both P-20 and P-200 are at different positions. Our P-1000 pipette has no decimal points. A micropipette is always used with a tip. Bigger, blue tips are for blue P1000 pipette and yellow, small tips for yellow P20 and P200 pipettes. You may load a tip by gently pressing the pipette on the tip and making sure that the tip is firmly attached to the pipette. Liquid will be drawn into the tip for dispensing. In order to use the pipettors accurately you must be familiar with the stops on the plunger. Press the plunger and feel for the resistance at the first stop. This is the stop you need to load the desired set amount of liquid in the tip. If you press the plunger beyond the first stop, you will feel the second stop, the plunger is pressed all the way down at this point. This is the stop you need to eject all the liquid from the tip. Practice pipetting ensuring you follow the guidelines described below. GUIDELINES FOR PIPETTING: 1. Selection of correct pipette Know the volume that you need to measure and select what pipette to use (P-20, P-200, P-1000). 2. Adjust the desired volume After selecting the pipette, adjust the volume by rotating the volume adjustment knob that is below the plunger. 3. Holding a pipette and loading a tip on it Always hold a pipette as shown in the picture, with the fingers holding the pipette and thumb being free to use the plunger. After adjusting volume and holding the pipette correctly, load the appropriate tip (Figure 8). 4. Loading the sample Once the tip is loaded, press the plunger to the first stop. This is the stop you need to load the desired set amount of liquid in the tip (The length of this stop will vary 6

7 depending upon the volume that has been set). Keeping the plunger pressed to the first stop, immerse the tip in the liquid (3-4 mm only, not too deep) to be loaded and SLOWLY release the plunger to allow the liquid to be taken up in the tip. Visually check the tip and make sure it has no air bubble in it. 5. Discharging out the liquid in the tip. To discharge out the liquid in the desired tube or container, SLOWLY press the plunger beyond the first resistance stop, to the second stop. This will ensure all the liquid in the tip has been discharged. DO NOT RELEASE the plunger while the pipette is still in the discharging container. Remove the pipette from the discharging container and then release the plunger. Doing so will avoid air bubbles in the container that you just dispensed your liquid in. 6. Discard the tip. After using a tip, the tip must be discarded by pressing the U-shaped tip discard slider (Figure 5). Always discard tip in an appropriate tip disposal container. RULES FOR PIPETTING: 1. Handle the pipette gently. 2. Never exceed the volume range of the pipette, either on lower or higher end. 3. If there is an option to pipette the desired volume using 2 different sized pipettors always use the smallest pipet for greatest accuracy. 4. Never invert a pipette, especially when it is loaded with liquid! Doing so will contaminate the pipette. 5. When withdrawing and dispensing liquids, always press ad release the plunger slowly. 6. Use a new tip for different liquids to avoid contamination. Do not touch the end of a tip. Keep the tip box closed when not in use. It is important to note that 1ml of water is equal to 1 g of water. This conversion allows us to confirm the volume of water dispensed by weighing it and allows us to assess our accuracy at pipetting. For example, if 860 µl of water is drawn up, when dispensed on weigh boat on the balance (ensuring the balance is first zeroed) the weight recorded should be g. A weight that does not match the expected weight indicates that an incorrect volume was drawn up and further practice is required. How much would 1 µl of water weigh? Pipet the volumes of water listed in the table into a weigh boat on the balance and determine by weight if you have pipetted the correct volume. Each person needs to complete their own table. Take turns using the pipettes and balance. 7

8 Activity 4: Temperature Most people in the United States use the degree Fahrenheit ( F) to measure temperature. In science and most nations worldwide the degree Celsius ( C) is the basic unit of temperature. There are several common temperatures that you should know in Celsius (Table 4). Use the provided thermometer to measure the temperature for the following: What is the room temperature of the lab in Celsius? What was the temperature of the ice bath in Celsius? What was the temperature of the water on the hot plate in Celsius? 8

9 Pre- Lab Activity Prior to this lab you should: 1. Read BioSkill 1: Using the Metric System and Significant Figures of your Biological Science textbook (Freeman, 6th edition pages 19-21). 2. Watch the video Mastering Micropipettes on proper pipetting technique. ( 3. Complete the following questions before coming to class. A) What is the base metric unit of mass? What is the base metric unit of length? What is the base metric unit of volume? B) Look at Table 5 to get an idea of how much some familiar items weigh. With these weights in mind, how much you think your smart phone and a pencil weighs. Estimated weight of your smart phone: Estimated weight of your pencil: Table 5. Weights of various items. Peacock Feather 0.8 g Slice of Bread 28 g Candy Bar 100 g Football 420 g C) Convert 77g to oz (1 gram = ounces) Convert 1510 mg to oz. D) Convert the length of a 6 inch Subway sub to meters (1 meter = inches). What is the length of the sub in centimeters? What is the length of the sub in millimeters? E) How many milliliters are in a 1.93 ounce bottle of 5-hour Energy (1 ml = fluid ounces)? How many liters are in a 1.93 ounce bottle of 5-hour Energy? 9

10 Post-Lab Activity The following problems will help you practice and gain a greater proficiency with the metrics system. Restate the following numbers using proper scientific notation. Example: 10,000 = 1x ,000 = 2. 1,300,000 = 3. 5,500,000,000 = = = = Complete the basic metric conversions m = km cm mm µm nm 8. I L = ml µl 9. 1 g = kg mg µg m = km cm mm µm nm mm = cm m µm nm µm = mm m nm cm = mm m mm = cm µm m µl = ml l ml = l µl ml = l µl ml = µl l mg = g µg g = kg mg kg = g g = mg Complete the following mass/volume equivalents µl = mg g ml = mg g µl = mg g µl = mg g ml ml = mg g µl 10

11 Determine the volume of fluid of each graduated cylinder. 28. ml 29. ml Determine the length of each black bar. 30. cm 31. mm 32. cm 33. mm Determine the appropriate temperature to record for the following sample (remember to include units)