Experiment #1 – Measurements
Name: _______________________________ Lab #1 Lab measurements
Objectives
1. Use measuring tools correctly.
2. Read and record measurements correctly (significant digits and unit).
3. Associate metric units with the quantities being measured.
4. Determine the density of a solid.
Introduction
Chemistry is an experimental science. It depends on careful observation and the use of good laboratory techniques. In this exercise you will become familiar with some basic operations that will help you throughout the course. Your success in future experiments will depend upon your mastering these fundamental operations. Because every measurement made in the laboratory is really an approximation, it is important that the numbers you record reflect the uncertainty of the device you are using to make the measurement. The assumption is made in this course that an uncertainty of at least one unit exists in the last digit.
10.01 mL could actually be 10.00 mL or 10.02 mL
Significant Figures and Metric Conversions
Significant figures are important in describing the accuracy of a measurement. The following rules paraphrased from your textbook will help you determine the number of significant figures in a measurement.
1. All nonzero digits are significant. 3.467 cm has 4 sig figs
2. Zeros between nonzero digits are significant. 2.07 mL has 3 sig figs
3. Zeros beyond the decimal point at the end of a number are significant. 6.00 mL has 3 sig figs
4. Zeros preceding the first nonzero digit in a number are not significant. 0.0341 g has 3 sig figs
5. The number of sig figs is the number of digits shown when the number is written in scientific notation.
500 g written expressing 3 sig figs would = 5.00 x 102 g
500 g written expressing 2 sig figs would = 5.0 x 102 g
500 g written expressing 1 sig fig would = 5 x 102 g
In science the metric system is used almost exclusively. The basic unit of length is the meter (m); of mass, the gram (g); of volume, the liter (l). The meter, the gram, and the liter are related to larger and smaller units by multiples of 10. Prefixes are used for the unit of measure expressing these multiples of 10.
Prefix Abbreviation Value x basic unit Some Conversion Factors
Kilo- k 1000 1 km = 1000 m
Deci- d 0.1 1 m = 10 dm & 1 km = 10,000 dm
Centi- c 0.01 1 m = 100 cm & 1 dm = 10 cm
Milli- m 0.001 1 m = 1000 mm & 1 cm = 10 mm
How does 1 kilometer compare to the other units in the metric system?
1 km = 1000 meters = 10,000 dm = 100,000 cm = 1,000,000 mm
Exact Numbers and Conversion Factors
Some numbers by their very nature are exact numbers. When you say you are holding five beakers in your hand, you mean exactly five beakers, not six or four. Thus, if this figure 5 is used in a calculation, you may regard it as having as many significant figures as desired: 5.0000… (an unlimited number). Another example of an exact number is a number used to relate quantities (conversion factors) within the same system of units: 1.0000… m = 100.0000… cm. These are defined values and possess an unlimited number of significant figures.
Sometimes it is necessary to convert from the British system to the metric system or vice versa. These relationships between units in two different systems are not exact numbers. These are the most common conversions utilized by students in this course to convert from the British system to the metric system or vice versa.
1 pound (lb.) = 454 grams (g) 1 inch (in) = 2.54 centimeters (cm)
1 quart (qt) = 946 milliliters (mL)
Temperature
Most thermometers are based upon the principle that liquid expands when heated. Our thermometers use alcohol as the liquid and are constructed so that a uniform-diameter capillary tube surrounds the alcohol reservoir. The thermometer is calibrated by defining two reference points: (1) the normal freezing point of pure water (0 oC, 32 oF ), and (2) the normal boiling point of pure water (100 oC, 212 oF). There are 100o between these two points on the Celsius temperature scale and 180o between these two points on the Fahrenheit scale. 100/180 = 5/9 and this is how the conversion factors were derived for the temperature conversion formula below. The simple act of taking temperatures with a thermometer can easily involve errors. When measuring the temperature of a liquid observe the following procedures:
1. Hold the thermometer away from the walls and bottom of the container.
2. Allow sufficient time for the thermometer to reach thermal equilibrium.
3. Be sure the liquid is adequately mixed.
4. Temperature measurements should be estimated and recorded to the nearest 0.1 0C. This is the accuracy of the thermometer.
When converting from degrees Fahrenheit to Celsius, make use of the following formula:
(0F – 32oF) x 100oC/180oF = oC
Sample problem: Convert the measured temperature of 70.0o F to degrees Celsius.
(70.0oF – 32.0oF) x 100oC/180oF = 21.1 oC 3 sig figs
When converting from Celsius to Fahrenheit use the following formula;
(oC x 1800F/100oC) + 32oF = oF
Note that the final answer is 21.1 oC because the initial measured temperature (70.0oF) was to the tenths place and had only 3 significant figures. The 100oC, 180oF and 32oC are all exact numbers and do not determine the significant figures. When using conversion factors the number of significant figures in the answer always comes from the measured quantity in an experiment or the given quantity in a problem.
Mass
The directions in this experiment are written for use with a digital pan balance. These balances give a mass measurement to the tenth place ( 0.1 g). This is considered the accuracy of the balance. Even though this is a digital instrument, the balance is making an estimate at the hundredths place. All measurements recorded in your data sheet should reflect this degree of precision. When using the analytical balance please be gentle with these sensitive instruments and use the following procedures:
1. Before placing your item to be massed on the balance check the display to make sure it is reading zero. If not press the tare bar once to zero out the balance.
2. Carefully place the item on the pan .
3. Allow sufficient time for the mass to be measured.
4. Record the mass, remove the item.
5. Never place chemicals directly on the balance pan; first place them on a weighing boat or in a container and then place the boat or container on the balance pan.
6. Clean up any materials you spill on or around the balance.
7. Never make any adjustments to the balance. If it seems out of order tell your instructor.
Length – metric ruler
The standard unit of length is the meter (m), which is 39.37 inches in length. Since you can estimate the measurement to the hundredths place with your rulers, all measurements recorded in your data sheet should reflect this degree of accuracy (0.01).
Volume
The graduated cylinder is the device you will be using in lab to measure liquid volumes. Graduated cylinders are tall, cylindrical vessels with graduations scribed along the side of the cylinder. Since volumes are measured in these cylinders by measuring the height of liquid, it is crucial that the cylinder have a uniform diameter along its entire height, which is the case with the glass cylinders you are using in lab.
Beakers and flasks are marked to indicate only approximate volumes and should not be used to measure volumes during experiments.
When measuring a volume in a graduated cylinder, read the point on the graduated scale that coincides with the bottom of the curved surface, called the meniscus, of the liquid. Volumes measured in the graduated cylinder should be estimated and recorded to the correct number of significant figures.
The volume of a solid object can also be measured using the graduated cylinder by water displacement. Water displacement is filling a graduated cylinder with enough water to completely cover the solid object, when it is placed into the cylinder. Be sure to place a rubber stopper in the bottom of the graduated cylinder to prevent the solid object from cracking the bottom of the cylinder. The difference between the measured volume of water before and after adding the solid object is the volume of that object.
Procedure:
Refer to the Experiment #2 Report to record all data. The data sheet corresponds to the procedure in letter and number. You will be graded based on accuracy of the values recorded, accuracy expressed in the correct number of significant figures in your measurement and neatness in presentation of the lab report.
Traditionally a pen is used to record data in the laboratory. If you make an error in recording your data simply draw a single line through your mistake and rewrite it correctly.
A. Mass
1. Describe the precision of the analytical balance.
2. Be sure to zero out the balance and use the same balance throughout your massing. Place a clean, dry 150 mL beaker on the pan of the balance, read and record the mass of the beaker.
3. Do the same with the 150 mL Wheaton jar.
4. Do the same with the metallic cylinder.
5. Do the same with a metallic cylinder.
6. Do the same with a weighing boat.
7. Carefully add approximately 2.5 grams of NaCl to the weighing boat while it is still on the pan in the balance. Read and record the mass of the weighing boat with the NaCl. Save this for part B.
8. Mass the second weighing boat.
9. Leave the second weighing boat on the balance and use the tare bar to zero the balance. Now add approximately 2.5 grams of NaCl to the boat and read and record the mass of only the NaCl. Save this for Part B.
B. Temperature
1. Describe the precision of the thermometer.
2. Fill a 600 mL beaker half full of tap water. Place your thermometer in the beaker and give it a minute to reach thermal equilibrium. Keeping the thermometer in the water and holding the tip of the thermometer away from the sides of the beaker, read and record the temperature.
3. Fill a 250 mL beaker half-full and place on a hot plate. Heat the water to boiling. Read and record the temperature of the boiling water, being sure to hold the thermometer away from the bottom of the beaker.
4. Fill a 400 mL beaker one-fourth full of tap water and add a 150 mL beaker full of crushed ice. Leave this set up for five minutes. Without stirring, place the thermometer in the beaker, resting on the bottom. Wait at least one minute and then read the temperature.
5. Take the two weighing boats from Part A and add the NaCl from each of them to the ice-water mixture in step #4 above. Stir the mixture for 1 minute and add more ice if needed. Read and record the temperature.
C. Length
Using a metric ruler, make the following measurements in centimeters.
1. Describe the precision of the metric ruler.
2. Measure the external height of a 600 mL beaker.
3. Measure the total length of a small test tube.
D. Volume
1. Fill a small test tube to the brim with water. Transfer the water to a graduated cylinder, measure, and record the volume of water.
2. Fill a large test tube to the brim with water. Transfer the water to a graduated cylinder, measure and record the volume of water.
3. Fill a 150 mL Wheaton jar flask to the brim with water, measure and record the volume of water.
4. Measure 5.0 mL of water and pour into the small test tube. Use the metric ruler to measure and record the height in centimeters.
5. Use the non-metal cylinder you massed in Part A and determine the volume of this solid object using the volume displacement method.
6. Use the metal cylinder you massed in Part A and determine the volume of this solid object using the volume displacement method.
Name _______________________ Section ______ Lab Partner __________________
CHM 130LL: Report for Experiment #2 – Measurements
Read the appropriate section of the lab introduction or procedure before starting each section.
A. Mass
1. Describe the precision of the analytical balance ___________________ g
2. Mass of a clean, dry 150 mL beaker ___________________ g
3. Mass of a 150 mL Wheaton jar ___________________ g
4. Mass of the non-metal cylinder ___________________ g
5. Mass of the metal cylinder ___________________ g
6. Mass of a weighing boat ___________________ g
7. Mass of the weighing boat with the first sample of NaCl ___________________ g
8. Mass of the NaCl ___________________ g
9. Mass of only the second sample of NaCl placed in the tared weighing boat ______________ g
B. Temperature
1. Describe the precision of the thermometer ___________ oC
2. Temperature of the tap water ___________ oC
3. Temperature of the boiling water ___________ oC
4. Temperature of water with crushed ice ___________ oC
5. Temperature of water with approximately 5 grams of NaCl ___________ oC
C. Length
1. Describe the precision of the metric ruler ___________ cm
2. Measure the external height of a 600 mL beaker ___________ cm
3. Measure the total length of a test tube ___________ cm
D. Volume
1. Volume of a small test tube ___________ mL
2. Volume of a large test tube ___________ mL
3. Volume of a 150 mL Wheaton jar ___________ mL
4. Height in centimeters of 5.0 mL of water in a small test tube ___________ cm
5. Volume of the non-metal cylinder you massed in part A ___________ mL
6. Volume of the metal cylinder used in part A ___________ mL
Questions and Problems – Show calculation setups and answers for the following problems.
1. Convert 23.5 oC to Fahrenheit and Kelvin ___________ oF
___________ K
2. Convert 65.8 oF to Celsius and Kelvin ___________ oC
___________ K
Use dimensional analysis (unit conversion) for solving problems 3-5.
3. Convert the mass of the 125 mL flask from grams to kilograms ___________ kg
4. Convert the mass of the weighing boat from grams to centigrams ___________ cg
5. Convert the volume of the small test tube from mL to liters. ___________ L
How many significant figures are in each of the following?
6. 1.234 cm _____ sf 7. 0.016 g _____ sf 8. 104 L _____ sf 9. 0.05600 ______ sf
Write the following numbers in scientific notation:
10. 1234 m = ________________________ m 11. 0.01234 m = _________________________ m
The definition of density is the mass of an object divided by the volume of the object.
Density = mass / volume
11. Use your data in this experiment to determine the density of the non-metal cylinder. Check the number of significant figures in each measurement and follow the rules of calculating with significant figures given in lecture.
_______________ g / mL
12. Use your data in this experiment to determine the density of the metal cylinder. Check the number of significant figures in each measurement and follow the rules of calculating with significant figures given in lecture.
_______________ g / mL
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