chem 1405 homework lav about precision

PRECISION

Pre-Lab Questions

  1. Label the place value names in the following measurement (the “ones” and “tenths” columns have been labeled for you):

563.123 g

ones

tenths

  1. In each of the following measurements, circle the digit that was estimated.
  • 9.468 g
  • 2.71 x 103 mol
  • When making a measurement using the centimeter ruler shown below, the estimated digit should be written in which decimal place?
  • Anytime a measurement is written, the precision of the equipment is apparent to the reader. Match measurements (a), (b) and (c) to the glassware scale you believe was used to make the measurement. Write the letter of the measurement (a, b or c) underneath the correct scale.
  • Read the following measurements to the correct number of significant figures. Remember to include units.

  • 25 mL
  • 25.1 mL
  • 25.12 mL

30

20

26

25

20

30

  • Balance read in grams_____________________________________________
  • Thermometer read in degrees celcius ________________________________________
  • Graduated cylinder read in milliliters_______________________________________

Name: ______________________________Date: ________________ Section: ____________

PRECISION

Data, Results and Observations

A. Investigating the Precision of Glassware, Part I

250 mL

Beaker

100 mL Graduated

Cylinder

10 mL Graduated

Cylinder

Mass of glassware, g

Mass of glassware + H2O, g

Mass of H2O, g (graph the value)

Volume of H2O, mL (graph the value)

B. Investigating the Precision of Glassware, Part II

150 mL Beaker

100 mL Graduated Cylinder

10 mL Graduated Cylinder

Initial volume, mL

Final volume, mL

C. Calculators and Measurements, Part I

100 mL Graduated Cylinder

Show your work here:

Initial water volume, mL

Volume with 1 marble, mL

Volume of 1 marble, mL

Volume with 7 marbles, mL

Volume of 7 marbles, mL

Average volume of 1 marble. mL

Do not round your calculator’s answer.

D. Calculators and Measurements, Part II

100 mL Graduated Cylinder

10 mL Graduated Cylinder

Total

Volume reading, mL

Calculated volume

Data Interpretation

1. Identify the decimal place (i.e. tens, ones, tenths, hundredths, etc.) and number of significant figures you should have for measurements for the volume and mass of water from Part A.

Decimal Place

Significant Figures

Volume measurement

250 mL beaker

Volume measurement

100 mL cylinder

Volume measurement

10 mL cylinder

Mass measurement

Balance

2. Compare the three sets of data represented by the lines on the class graph and answer the following questions.

  • When you look at each line, what do you notice about the spacing of the points?
  • How did the glassware used to make the measurements affect the outcome of the graphs?
  • What does this tell you about the precision of the measurements for the different pieces of glassware?
  • Rank the three pieces of glassware from highest to lowest amount of precision in their measurements.
  • What is the relationship between accuracy and precision? Are you assured an accurate measurement if you use a more precise piece of glassware?

3. How did the precision of the glassware you used affect your ability to see the change in volume in “Part B. Investigating the Precision of Glassware, Part II”?

4. If an experiment called for adding approximately 75 mL of acid to the reaction, which would be the appropriate piece of glassware to use? Why?

5. If an experiment called for adding 75.0 mL of acid to the reaction, which would be the appropriate piece of glassware to use? Why?

6. Examine your data and calculations for the determination of the volume of the marbles.

  • How many significant figures should you have for your determination of the volume of 1 marble?
  • How many significant figures should you have for your determination of the volume of a marble when using 7 marbles?
  • What would you expect to happen to the significant figures for the volume if you had used 70 or 700 marbles? Why?
  • Is the volume on your calculator display representative of the precision of the equipment used to make the measurements? Why or why not?
  • Using the data you have for the total volume you calculated and the total volume you observed for “Part D. Calculators and Measurements, Part II”, explain why the rule for significant figures in addition and subtraction is to keep the same number of decimal places as the number that originally had the fewest number of decimal places. HINT: Think about the level of precision you have with each piece of glassware.