Purpose: To investigate uncertainty, precision, and accuracy in scientific measurements.
Pre-lab:
1) Look up the definition of precision and accuracy in your textbook and write down the definitions.
2) Draw a data table on your paper like Table I at the bottom of this page.
Note: This experiment requires careful attention in making measurements.
?
PLEASE DO NOT WRITE ON THIS HANDOUT!
Procedure:
Step 1: Fill 4 different sized graduated cylinders to capacity with tap water. So that the 10 mL cylinder contains exactly 10 mL of water, the 25 cylinder contains 25 mL of water, the
50 mL cylinder contains 50 mL of water, and the 100 mL cylinder contains 100 mL of water.
Use the pipet to remove water or add water in order to get optimum instrument readings.
Step 2: Weigh a dry 100 mL beaker carefully and record the mass in grams under “Mass of Beaker.”
Remove the beaker from the balance pan and do step 3.
Step 3: Using the pipet remove 5 mL of water from the 100 mL graduate cylinder and place it into the
100 mL beaker from step 2. Record the mass of water + beaker in data table I under “Mass of
Beaker”. Do not empty the beaker. Remove the beaker from the balance pan and do step 4.
Step 4: Using the pipet remove 5 mL of water from the 50 mL graduate cylinder and place it into the beaker along with the water from the previous step. Record the mass of water + beaker in data table
I under “Mass of Beaker.” Do not empty the beaker. Remove the beaker from the balance pan and do step 5.
Step 5: Using the pipet remove 5 mL of water from the 25 mL graduate cylinder and place it into the beaker along with the water form the previous step. Record the mass of water + beaker in data table
I under “Mass of Beaker”. Do not empty the beaker. Remove the beaker from the balance pan and do step 6.
Step 6: Using the pipet remove 5 mL of water from the 10 mL graduate cylinder and place it into the beaker along with the water from the previous step. Record the mass of water + beaker in data table
I under “Mass of Beaker.” Remove the beaker from the balance pan, empty the water from the beaker into the sink and dry the beaker. Go on to step 7.
Step 7: Determine the mass of water added to the beaker for each incremental addition of water on your copy of Table I. PLEASE DO NOT WRITE ON THIS HANDOUT!
Table I:
Cylinder Type
100 mL
50 mL
25 mL
10 mL
beaker + water Mass of beaker Mass of water alone
Chem Measurement Lab.cwk

Since the density of water is very close to 1.0 g/mL, you can find the volume of water added by each graduated cylinder by multiplying the mass of water times 1.0 mL/g. The volume of water determined by the balance is assumed ( by virtue of engineering design) to be more accurate than the graduated cylinder. Transfer information concerning the mass of water from Table I to Table II and complete Table II by performing the suggested calculations described at the heading of each column. The deviation is the absolute value of the expected ( 5.0 mL) minus the observed (what you measured) volume of water.
Table II:
Cylinder size Column 1
Mass of water in grams
Column 2
Mass (g) x 1.0 mL/g or volume (mL)
Column 3
Deviation
|exp-obs|
100 mL
50 mL
25 mL
10 mL
Examine the data in Table II. How accurate is your instrument? Record the volume as determined by using the balance from the 2nd column from Table II on the board. Record your data on the board; the first group up is group 1, the second to record data is group 2 and so forth.
Group Table III:
Group measurement
10 mL (5 mL) actual volume first group second group third group etc.
Class avg.
25 mL (5 mL) actual volume
50 mL (5 mL) actual volume
100 mL (5 mL) actual volume
Question 1: On average how accurate is each of the graduated cylinders? This is a way of asking how close did the instrument come to actually measuring 5.0 mL of water.
Question 2: How precise is each of the instruments? For example, how closely did your 10 mL cylinder measure come to other groups? (The agreement of data is not the same as how accurate the data was).
Question 3: What is uncertainty?
Question 4: How do recorded data readings reflect uncertainly in a measurement? Examine each graduate cylinder. How much precision is implied by the markings on each instrument?
Conclusion: Write a summary. Include generalizations about the precision and accuracy of each of the graduate cylinders used in this experiment. Distinguish between accuracy and precision. Which instruments did you feel most certain about while you were using them to measure 5.0 mL of water?
Chem Measurement Lab.cwk