Lab supply Marble You supply: String Empty 2-L Soda bottle with cap Metric ruler Corwin Textbook

Transcription

1 Experiment #02: Measurements, Metric System and Density of Irregular Object Objective: The purpose of this experiment is to become familiar with the metric system by taking measurements with metric units. A second purpose of this experiment is learn how to handle data collection and then and the proper use of significant figures, proper unit usage and the application of dimensional analysis. Equipment and Chemicals (Total Time min.) Lab Kit Contains: Alcohol Thermometer 100mL beaker 250 ml beakers 10 ml grad cylinder 50 ml graduated cylinder 100mL graduated cylinder Cork (for 20 x 150 mm) Lab supply Marble You supply: String Empty 2-L Soda bottle with cap Metric ruler Corwin Textbook Discussion Observations are very important in chemistry experiments. Two types of observations are qualitative and quantitative. Qualitative observations are description of an observable property such as the color, odor, or texture. Quantitative observations are numerical measurements and require a number and a unit in order to be complete. For example, if your height is six feet, then the number is "six", and the unit is "feet". In scientific work, quantitative measurements are almost always recorded in the units of the metric system. In laboratory work, the basic metric units of mass, length, and volume used are the gram (g), the meter (m), and the liter (L), respectively. Also commonly used in the laboratory is the degree Celsius ( C), which is the metric unit of temperature. For convenience and consistency, either multiplying or dividing by some power-of-ten subdivides metric units. The prefixes deci-, centi-, and milli-, mean 1/10, 1/100, and 1/1000 of the original unit, respectively. The prefix kilo- and mega-, means 1000 and 1,000,000 times the original unit. Refer to some tables (textbook, website ) containing the commonly used units and their conversions. It is very important to realize that whenever any measurement is made, some part of the measurement is an estimate. Therefore, any measurement always has some degree of uncertainty. For example, in measuring the diameter of a coin, as shown in Figure 1, a diameter as 1.52 cm would be recorded. The first two digits, one and five, would be the same regardless of who made the measurement; these digits are considered certain. The third digit, two, however, is determined by a visual estimate. This digit would depend on who made the measurement; that is, it is uncertain. Each number that is recorded, including all the certain numbers, and the first uncertain number, are called significant figures. The scale of the measuring device used determines the number of significant figures obtained in a measurement. Figure 1 Suppose that the outside diameter of a coin is to be measured using the centimeter scale. The diameter is recorded as 1.52 cm, the first two digits, 1.5, is read with certainty, and the third digit, 2, is doubtful or estimated with an uncertainty of perhaps 1 in that digit or cm overall. Only three digits are recorded because any more digits to the right would not be valid significant figures. Dimensional analysis is a technique used when converting measurements from one unit to another. For example, one might wish to convert a measurement from units of centimeters to meters, or liters to milliliters. This is accomplished by using a conversion factor. A conversion factor is a ratio that relates two units. A conversion factor is given by an equivalence statement, which defines the relationship between the units that is being converted from to the unit that is being converted to. In the example above the conversion factor relates inches and centimeters. What is the diameter of the above coin in inches? This problem may be expressed as:.52 cm =? in. The question mark stands for the number we would like to find. The relationship between centimeters and inches is set up using the equivalence statement: 2.54 cm = 1 in. This equivalence statement can lead to two possible conversion factors: 2.54cm 1 in or 1 in 2.54 cm

2 We choose the conversion factor that upon multiplication with the given measurement, will cancel the unit we want to convert from (cm), and leave behind the unit we want to convert to (inches). 1 in 1.52 cm = in 2.54 cm Two important facts about this conversion are: 1. When the units changed from centimeters to inches, notice that the numerical value also changed (from 1.52 to 0.598). That is, 1.52 cm is exactly the same value (i.e. same length) as in. 2. Notice that although your calculator will display more than three decimal places in the final answer, the final answer is reported to three significant figures. All answers should be rounded to the correct number of significant figures using the rules for rounding-off and significant figures. Sometimes, measurements may be combined to express a relationship between them (e.g. miles/gallon). One important property of a substance, density, is defined as the relationship of mass (g) and vol (ml): Density = mass (g) volume (ml) The density of a substance is a characteristic property of that particular substance which can help identify the material. There are two ways to determine the volume of a regularly shaped object when calculating its density. One is called directmeasurement, the other is the volume by displacement method. In the direct measurement method, the appropriate dimensions of the object are measured using either a metric ruler or a meter stick and then the dimensions are placed in the formula for the proper geometrical shape to calculate the volume. In the volume by displacement method, the object is submerged in water contained in a graduated cylinder. Submerging of this object under water will cause the water level to rise. The difference in the water level before and after the object is submerged is due to the volume of the object. (Figures 3 & 4.) The density of a substance is a characteristic property of that particular substance which can help identify the material. Substance Formula Density (g/cc) Air Ethanol C 2 H 5 OH Water H 2 O 1.00 Glass SiO Aluminum Al 2.70 Iron Fe 7.86 Silver Ag 10.5 Lead Pb Mercury Hg Gold Au 19.3 There are two ways to determine the volume of a regularly shaped object when calculating its density. One is called the direct measurement, the other is the volume by displacement method. In the direct measurement method, the appropriate dimensions of the object are measured using either a metric ruler or a meter stick and then the dimensions are placed in the formula for the proper geometrical shape to calculate the volume. In the volume by displacement method, the object is submerged in water contained in a graduated cylinder. Submerging of this object under water will cause the water level to rise. The difference in the water level before and after the object is submerged is due to the volume of the object. (Figures 3 & 4.) Special Notes for Experiment: The emphasis of this lab will be on proper use and reading of the measuring device. In the work up of the date, emphasis will be place on the proper use of significant figures and unit conversion. After this lab you should know the difference between accuracy, precision and significant figures in a measurement. You will also become more familiar with the metric system. When showing calculations show clearly how one unit is converted to another and then write your units for each answer.

3 Procedure 1. Measuring Temperature Fill a 250 ml beaker 3/4 full (175 ml) with tap water and allow it to reach room temperature (about 30 minutes). You may proceed to steps 2-4 while you wait. After 30 minutes, use an alcohol thermometer to measure the temperature of the water to the correct number of significant figures as determined by the calibration marks of the thermometer, see Figure 2. Record the temperature in your worksheet. (Digital Picture1: Take a digital picture of thermometer in the 250mL beaker of water.) Figure 2 Note that the thermometer is calibrated to the nearest 0.5 C and that the end of the alcohol column lies between 21.0 C and 21.5 C. Thus, there is no doubt about the first two digits, 21. The only remaining task is to estimate the distance between 21.0 and 21.5 and to record one doubtful or estimated digit. The estimated digit is probably 4, and the temperature is recorded as 21.4 C. This conveys to the readers that the uncertainty in the temperature is at least C. Obviously, it would be foolish to record the temperature to the hundredth place i.e., C, because this digit, in this case the digit 5, has no significance 2. Measuring Mass Measure the mass of the following solid objects using the laboratory digital scale and record your data in your data sheet with the correct number of significant figures and units. Note that all the numbers in the digital scale reading are significant. a. Cork (for 20 x 150 mm) b. 100 ml beaker c. 2-L soda bottle empty with cap. d. Now estimate the mass of your textbook and record your estimate in your data sheet. DO NOT WEIGH THE OBJECT on the scale!! (Digital Picture2: Take a digital picture of the 150 ml beaker on the scale with the mass clearly displayed.) 3. Measuring Length Measure the following lengths (in centimeters), using a meter stick, and record the data in your data sheet with the correct number of significant figures and units. Record the measurement as precise as the instrument allows. a. Your height in centimeter b. your foot c. circumference of the 2-L soda bottle. d. Now estimate the length (in centimeters) of this lab book and record this estimate in your data sheet. DO NOT MEASURE THE LENGTH! (Digital Picture3: Take a digital picture of string around the 2-L soda bottle and the metric ruler next to the bottle.)

4 4. Measuring Volume Fill a 100 ml, 25 ml and 10 ml graduated cylinder halfway up with water. Read the volume (in milliliters) of each of the three graduated cylinders at your laboratory table and record the data in your notebook. (See Figure 3 for instructions in reading the volume of a fluid in a graduated cylinder.) a. 100 ml graduated cylinder b. 50 ml graduated cylinder c. 10 ml graduated cylinder d. Now fill the 20 x 150 mm test tube so that the meniscus touches the bottom of the KIMAX label printed on the side of the test tube. Estimate this volume. DO NOT TRY TO MEASURE THE PRECISE VOLUME BY POURING INTO A GRAD CUYLINDER! (Digital Picture4: Take a digital picture of all three grad cylinder with the liquid and the test tube with the water filled to the KIMAX mark) Figure 3 When reading a graduated cylinder, read the bottom of the meniscus while viewing the meniscus at eye level. In the figure, the volume reads 53.9 ml. Note the precision of the graduated cylinder is to the tenth of a ml. ( ml) Figure 3 5. Determination of Density a. Take the glass marble that was issued to you. Determine the density using the method of direct measurement (geometry) and by volume displacement. In the direct method, measure the mass using the digital pocket scale and the volume by using the dimensions of the sphere (volume = 4/3 x π x r 3 ), where π is 3.14 and r is radius of the sphere or the diameter 2. For example if a sphere has a diameter of 2.6 cm, then the volume is calculated to be 4/3 x 3.14 x = 9.20 cm 3. Record your work and data in your data sheet. b. Using the same solid, determine the volume by water displacement (see Figure 4). Using the mass from part 5a, calculate the density. (Digital Picture5: Take a digital picture of the object submerged in water in the 100mL graduated cylinder.) Record your work and data in your data sheet. (density by displacement) c. How do these two values for densities compare? Explain why they might differ. Figure 3 The density of an regular shaped solid (spherical marble) can be found by first determining the mass of the solid and then placing the solid in a graduated cylinder partly filled with water (or some liquid in which the solid does not float). The solid will displace a volume of liquid equal to its own value. Thus, by noting the positions of the meniscus before and after the addition of the solid, the volume of the solid can be determine. Figure 4 Write the conversion factor on the heading of each table for this section. 6. Mass Conversions Convert the masses recorded in Step 2 (a-c) from grams to milligrams, kilograms and pounds. Record your work and data in your notebook. Link to Unit conversion 7. Length Conversions Convert the lengths recorded in Step 3 (a-c) from centimeters to millimeters, meters and inches. Record your work and data in your notebook. 8. Volume Conversions Convert the volumes recorded in Step 4 (a-c) from milliliters to liters and gallons. Record your work and data in your notebook.

5 Experiment #02: Measurements, Metric System and Density of Irregular Object Name Lab Partner Date Measurement Reading 1. Temperature of water in 250 ml beaker (use correct number of significant figures and correct units). Temperature of water: 2. Measuring Mass (use correct number of significant figures with correct units). a. Cork (for 20 x 150 mm): b. 100 ml beaker: c. 2-L soda bottle empty with cap: d. Course textbook (estimate): 3. Measuring Length (use correct number of significant figures with correct units). a. your height: b. your foot: c. circumference of 2-L bottle: d. length of course textbook (estimate): 4. Measuring Volume (use correct number of significant figures with correct units). a. 100 ml graduated cylinder: b. 50 ml graduated cylinder: c. 10 ml graduated cylinder: d. test tube (estimated volume):

6 5. Determination of Density a. Density by Geometry: Show your work for part 5a. mass dimension: length width height volume of cube density b. Density by Displacement Show your work for part 5b. mass volume initial volume final Volume solid density c. Explain why the two density values above may be different. 6. the digital pictures of the following: 1. Digital Picture1: Picture of thermometer in the 250mL beaker of water. 2. Digital Picture2: Picture of the 100 ml beaker on the scale with the mass clearly displayed 3. Digital Picture3: Picture of string around the 2-L soda bottle and the metric ruler next to the bottle. 4. Digital Picture4: Picture of all three graduated cylinder w/ liquid & test tube filled to the KIMAX mark. 5. Digital Picture5: Take a digital picture of the object submerged in water in the 100mL graduated cylinder.)

7 7. Unit Conversions Mass Conversions Using your data from steps 2 (a-c), show a sample calculation for one of each type of conversion. (Be sure to write the conversion factor you will use above each column.) mg = g g = kg lb = g measurement from 2 milligrams kilograms pounds a). cork (g) a a a b). beaker (g) b b b c). 2L bottle (g) c c c Show sample calculation here (or behind this page): 7. Length Conversions Convert the lengths recorded in steps 3 (a-c) from centimeters to millimeters, meters and inches. (Be sure to write the conversion factor you will use above each column.) mm = cm m = cm in = cm measurement from 3 millimeters meters inches a) height (cm) a a a b) foot (cm) b b b c) 2L bottle (cm) c c c Show sample calculation here (or behind this page): 8. Volume Conversions Convert the volumes recorded in Step 4 (a-c) from milliliters to liters and gallons. (Be sure to write the conversion factor you will use above each column.) ml = L gal = L measurement from 4 Liters gallons a) 100 ml cylinder (ml) a a b) 50 ml cylinder (ml) b b c) 10 ml cylinder (ml) c c Show sample calculation here (or behind this page):