LABORATORY MEASUREMENT - THE METRIC SYSTEM
(1) To learn to manipulate laboratory instruments to measure length, mass, volume, count, and time.
(2) To introduce the relationship of the metric units to English units.
(3) To learn the difference between precision and accuracy.
(4) To learn to calculate a deviation and a per cent error in experimentation.
In our everyday world we are faced with
many measurements. For each measurement there
must be a UNIT of measure. Some are
simple, some are very complicated. After
12 years of schooling most students know how a mile gallon, pound, foot, inch,
yard quart and ounce relate to everyday life.
There are all measures of the ENGLISH system of measurement, which
system is not used in
Have you heard of a CALORIE? KILOWATT-HOUR? On your JEA bill electricity is measured in kilowatt-hours, water in cubic feet. What does 55 miles/hour really mean. Pollution is measured in Parts per Million (ppm) or Parts per Billion (ppb). These are multiple dimension measures.
THERE ARE 5 CLASSIFICATIONS OF SINGLE (SIMPLE) DIMENSION UNITS. They are: LENGTH MASS COUNT TIME VOLUME (length cubed)
All other measures are combinations of these simple dimensions.
Speed or Velocity is length per time;
Acceleration is change in velocity per unit of time:
Force is mass times acceleration:
Work is force times distance moved:
Power is work per time:
In freshman chemistry, students seldom advance beyond one or two dimension measures. Some of the two dimension measures to be calculated in later lab experiments are: Density, % Recovered, Molecular Mass, Mole Ratio, Gram Molar Volume Constant, % Purity, Molarity, Molality, % by Mass, and Normality (no longer used).
PART 1: LENGTH
You will work with a lab partner only on this part of the lab.
1. Obtain a two meter stick from the equipment cart of two one meter sticks. Tape a sheet of paper to the wall. Mark your height on the piece of paper.
2. Measure your height in inches and in centimeters (each person) using either two meter sticks or the special two meter stick. Record the data using at least 3 significant digits.
3. Calculate the number of centimeters in one inch by dividing your height in centimeters by your height in inches.
or 1.00 in. = 2.59 cm.
4. The True Value is 1.0000 in. = 2.5400 cm (exact). Find the difference between the true value and your experimental value. This is called the deviation.
Example: Experimental value 2.59 cm/in
True value -2.54 cm/in
Deviation .05 cm/in
5. Calculate your percent error using the formula:
% Error =
Example: % Error =
= 2 %
6. Now measure your height in centimeters and inches using only a one meter stick. (You will have to move the stick to make the measure; this may introduce error.)
7. Again calculate the number of centimeters in one inch, the deviation from the true value, and the % error.
PART 2: FINDING VOLUME FROM LENGTH
8. Obtain an object of unknown volume from the lab cart (Block of wood, metal bar, or your textbook. (Record the number of this object, if labeled or description).
9. Measure the length, width, and height of the object to the nearest 0.1cm and to the nearest 0.125 inch (one eighth) or 0.0625 inch (one sixteenth) using a ruler.
10. Record the data and calculate the volume using the formula (both cm3 and in3):
Volume = length x width x height. V = _______ cm3 = ________ in3
The correct value is 16.48 cm3 = 1.000 in3
PART 3: MASS
In the laboratory we use several devices to measure mass. They are called balances. For large masses we use triple beam balances. Below are two sets of directions for the two main types of triple beam balances.
Thought question: Why are they called triple beams?
TRIPLE BEAM BALANCE INSTRUCTIONS:
A. With the pan empty, adjust all weights to zero. Be certain all weights are in the zero groove.
B. Use the damper button to bring the pointer to a steady position. Depress the damper. If the POINTER is not on the center line, adjust the ADJUSTMENT Screw until the pointer is centered on the middle line.
C. Load the balance.
D. Move the 100 gram weights until the pointer drops below the center line. You have set 100 too many grams. Move the 100 gram weight back one position. If the object exceeds 500, hang the weights in the hole outside the end of the POINTER, reset to zero and start again.
E. Repeat step D with the 10 gram weights.
F. Slide the gram weight until the POINTER is centered.
G. Read the weight to the nearest 1/10 gram.
3. Use an object such as your car keys to calculate the number of grams in one ounce by first using a triple beam balance and some balance that will record masses in ounces. If there is no device, skip this part of the experiment.
Mass of Object = __________g (triple beam)
Mass of Object = __________oz (some ounce measuring device, if available)
4. The true value is 1 oz. = 28.35 g. Calculate the deviation and percent error.
5. Obtain an unknown mass of less than 100 grams from the lab cart. Record its number.
6. Find the mass of the unknown using the triple beam balance. Record your data to the nearest 0.1 g.
7, Repeat steps 5 and 6 using a mass of greater than 1000 grams.
Have the lab instructor check your results.
Top Loading Balances (0.01 g on each island):
For smaller masses, we use top loading balances and analytical balances.
Over the years the United States Ming has changed the ratio of different metals used in making the various coins. The last change for the penny occurred in 1982.
Following is a brief chronology of the metal composition of the one-cent coin (penny):
Copper-Plated Zinc: 2.5% Cu, Balance: Zn 97.5 %
Mass: 2.500 g
Diameter: 0.750 in., 19.05 mm
Thickness: 1.55 mm Edge: Plain
1. Obtain three pre-1982 pennies and three post 1982 pennies. Weigh each penny on the 0.01g top loaders at each island in the lab.
__________g (6 pennies) Average Mass of One: _________g
__________g (13 pennies) Average Mass of One: _________g
__________g (25 pennies) Average Mass of One: _________g
Finding the True Value:
Balance Wheat Penny Zinc Penny
Triple Beam ____________g ____________g
Top Loader (0.01g) ____________g ____________g
Top Loader (0.001g) ____________g ____________g
(only one in the lab at the front desk)
Analytical Balance ____________g ____________g
(only one in the lab at the front desk)
Precision versus Accuracy:
Dart Board A Dart Board B Dart Board C
In the World of Chemistry Video: “Measurement” toward the end of the film there was a discussion of Accuracy and Precision in experimentation.
Precision: The agreement of repeated measurements of a quantity with another.
Accuracy: The agreement between the measured quantity and the accepted
(or true) value
In your post lab report, write a discussion of accurate and precise. How did the masses of the same type of penny vary?
Uncertainty: The degree on inexactness in a measurement obtained from
In your post lab report, write a discussion of exactness and uncertainty in laboratory measurements.
An element found in nature has a mass number, which is a whole number, and an atomic mass, which is a rational number. On the periodic chart, the masses of elements are reported as atomic masses, not mass numbers. Why?
PART 4: VOLUME
PROCEDURE FOR READING A GRADUATED CYLINDER
A. Note the two successive numbers on the graduations
the diagram shown to the left:
note the 50 and 45 numbers on the 50 mL graduate.
B. Count the spaces between the lines of 50 and 45.
In the diagram there are 5 spaces between 45 and 50.
C. Therefore C. The difference between 50 and 45 is 5 mL There are five spaces. Divide the number of milliliters by the number
of spaces: 5 ÷ 5 = 1 milliliter for each line. (± 0.1 mL)
D. When reading volumes, count the number of lines between the bottom of meniscus and the next lower number. In the diagram there are three lines between the meniscus and 40 ml line. Therefore the reading is between 43 and 44 milliliters.
E. Estimate the distance between the lines to estimate the 0.1 mL.
This reading is 43.8 ± 0.1 mL.
F. In the diagram to the right, proper eye
Alignment is a direct line of sight toward
This will avoid parallax error, This reading
Is 87.5 ± 0.1 mL
It is two line below 90 and seven lines above 80.
G. You instructor will demonstrate the dark line technique
(The dark line must be several lines below the meniscus)
2. Obtain one of the bottles (milk, soda pop or liquor) and fill each with water. Record the volume in ounces or quarts.
3. Using the appropriate size graduated cylinder, measure the volume of water contained in the bottle and record that volume.
4. Calculate the number of milliliters in one ounce. (63 oz. = ˝ gallon, 32 oz. = 1 qt)
5. The accepted value is 29.5 ml = 1 oz. Calculate your deviations and percent error.
6. Record the volumes of liquid in each of at least 6 different graduated cylinders on the instructors table or the side counter.
7. Record the cylinder numbers/capacity. Record the volumes using as many significant digits as each graduated cylinder allows.
Have the lab instructor check your results.
PART 5: COUNT
There are many counting units commonly used. Dollars and cents are counting units. Dozens, cases, six pack, each, pack, ream of paper, and gross are terms commonly used. (What is a “Baker’s” dozen?)
In chemistry we use the mole: 6.023 x 1023 objects = 1 mole. A mole is sometimes thought of as a “chemist’s dozen”.
In CHM 1025C or CHM 1032C (especially hospitals) you may encounter another counting unit, the EQUIVALENT. It is the first cousin to a mole, so to speak.
However, 20 years ago it was removed from the first year chemistry textbook.
Bean Jar or Gum Ball Experiment may be performed if time permits during this Measurement lab, otherwise during our first crucible experiment when we study mass relationships of chemical reactions we will attempt this experiment.