Chemistry Name:______
Accuracy and Precision in measurement
Objectives:
ü Define and understand precision and accuracy
ü Measure precisely
ü Write numbers in scientific notation
ü Convert between units
ü Calculate percentage error
Formulas:
D=m/v
Percentage Error= Experimental value-Accepted value x 100
Accepted Value
***Percentage error may either be positive or negative! If % error is negative this means that your value was lower than the accepted value. If you are less than 10% in error we would as a class consider that to be ACCURATE! J
Part 1: (answers will be written on last page!)
1. Examine the centimeter scale of the plastic ruler. What are the smallest increments?
2. You can measure one place value past the smallest increment. So, to what decimal place can you estimate to with this ruler? (ex. 1.0, 0.1, 0.01, 0.001)
3. The uncertainty in a measurement is +/- ½ the smallest increment. What is the uncertainty in a measurement made with your plastic ruler?
4. Using the ruler, measure the INSIDE diameter of the 100mL graduated cylinder.
5. Measure the INSIDE height of the cylinder to the 50mL mark (do not include the stand in your height measurement!). Record these measurements in data table #. Remember to measure to the precise number of decimal places that you answered to in question #1.
Part 2: (answers will be written on last page!)
1. Examine the graduations (increments) on a 25mL graduated cylinder, what place value can you estimate to? Record in data table
2. Examine the graduations (increments) on a 10mL graduated cylinder, what place value can you estimate to?
3. Examine the graduations (increments) on a 100mL graduated cylinder, what place value can you estimate to?
4. What is the point of having so many graduated cylinders (base this answer on your three previous observations)?
5. Using the balance, measure the MASS of the dry 25mL cylinder. Record the mass in Data Table #2.
6. Fill a beaker ½ full of water and measure its temperature (remember to look at the increments and estimate one place value past the smallest increment). Look up the density of water for this temperature in your textbook or online and record in Data Table #2 .
4. Fill your graduated cylinder with water to a level between 10mL and 25mL; read and record the volume to the correct number of decimal places. Measure the mass of the water plus the cylinder, then record this value in Data Table #2. SAVE the water in the graduated cylinder for Part 3.
Part 3: Measuring volume by the water displacement method.
1. Add a sufficient quantity of the assigned metal shot (aluminum, copper, or lead) to the cylinder containing the water (saved from part 2) to increase the volume by at least 5 mL. Measure the VOLUME and the MASS of the metal shot, water, and cylinder. Record your measurements in data Table #3.
2. Use subtraction to calculate the mass of the metal shot and the volume of the metal shot.
****Disposal: Wash out all of your glassware and dry it. Make sure that your table is dried off as well. Wash your hands when you are done and place your stools on your lab table.
Analysis:
*SHOW ALL OF YOUR WORK!!! OR…….you will RE-DO!
1. Calculate the volume of the graduated cylinder to the 50.0mL graduation (Hint: need a formula for the volume of this particular shape-see PLANNER!)
2. Assume the accepted value for the volume of the graduated cylinder is 50.0cm³. Calculate percentage error (see formula on front page).
3. Calculate the mass of water as measured by the balance (data table #2).
4. Calculate the mass of the water from its measured volume and its density (data table #2).
5. Using the mass of the water determined by the use of the balance as the accepted value (question 3) and the mass of water calculated from the density (question 4) calculate the percentage error .
6. Determine the volume of the metal, using your measurement of the volume of water displaced by the metal. (data table #3)
7. Using your measurements in Data Table #3, determined the mass of the metal.
8. Calculate the density of the metal. (formula on front page)
9. In a handbook, your textbook, or online, look up the density of the metal you used. Calculate the experimental error and percentage error for the density of the metal shot you determined in item 8. (In SI, the density of liquids and solids is equal to the specific gravity.)
10. Record below any values obtained by you and your classmates for the density of the SAME metal.
11. Calculate the average density of the five results of metal shot.
12. Calculate the percentage error for the average density of the metal shot.
13. What are the possible sources of experimental errors in this experiment (don’t say human error, be specific in something that your group did wrong, or something that you could have done wrong).
14. Sarah and Jamal determined the density of a liquid three times. The values they obtained were 2.84g/cm³, 2.85 g/cm³, and 2.80 g/cm³. The accepted value is known to be 2.40g/cm³.
a. Are the values that Sarah and Jamal determined precise? Explain.
b. Are the values accurate? Explain.
15. Not in this lab, but in many you will see numbers written in scientific notation. For example 3.0 x 104 or 5.5 x 10-5 . Under/Above each number write the corresponding number or scientific notation, whichever is needed. Remember, numbers less than 1 have negative exponents and numbers greater than 1 have positive exponents.
0.005 / ? / 5,680,000 / ?? / 3.0 x 108 / ? / 8.74 x 10-4
16. Convert the following units to a different unit by filling in the blank.
12 dozen eggs = ______single eggs and 657 seconds = ______minutes
Part 1
1.
2.
3.
Part 2
1.
2.
3.
DATA TABLE 1Inside diameter of graduated cylinder
Inside height of graduated cylinder
DATA TABLE 2 / measurement / units
Mass of empty graduated cylinder / g
Temperature of water / C
Density of water / g/cm³
Volume of water / mL
Mass of graduated cylinder and water / g
DATA TABLE 3 / measurement / units
Volume of water / mL
Mass of water + graduated cylinder / g
Volume of metal + water / mL
Mass of metal + water +graduated cylinder / g
Mass of the shot / g
Volume of the shot / mL
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