Chem: Accuracy and Precision and Significant Figures

Topics: Accuracy and precision, Significant figures, Measurement Pages: 54 (14806 words) Published: October 28, 2014
Introduction to Chemistry Laboratory:
A Lesson on Tools, Techniques and Measurements
PURPOSE: The purpose of this set of experiments (3 total) is to become familiar with the common types of laboratory glassware and equipment, and how to obtain and analyze data from these items. LEARNING OBJECTIVES: By the end of this experiment, the student should be able to demonstrate the following proficiencies:

1. Know which glassware (beakers, burettes, pipettes, graduated cylinders, flasks, etc) should be used in various circumstances.
2. Know how to “correctly” measure volume and mass (weight). 3. Become familiar with significant figures and its relationship to measurements and data recording (significant figures).
4. Become familiar with the errors, precision and accuracy associated the various measurement tools and techniques. 5. Determine the density of liquids and solids.
6. Determine the best-fit straight line as a method to examine linear relationships and to use this relationship as a predicative model such as in the determination of the percent copper and zinc in pennies based on density measurements.

7. Record laboratory data and observations.
Erlenmeyer Flasks
o 125 mL
o 250 mL
o 100 mL
graduated cylinders
o 10 mL
o 25mL
o 50 mL
Volumetric pipettes
o 10 mL
Measuring pipet
o 10 mL
Burette clamp and stand
Various liquids and solids for density determination measurements o Liquids
Distilled water
Carbon Tetrachloride
o Solids
Laboratory glassware. There are two major categories of laboratory glassware: (1) those that contain a certain volume (volumetric flasks) and (2) those that deliver a certain volume (pipets, burets, and graduated cylinders). “To Contain” glassware (sometimes labeled TC) is typically used for preparing solutions of known volume. “To Deliver” glassware (sometimes labeled TD) is used to transfer known volumes between containers. 1

Some glassware is very carefully designed and marked for high accuracy/precision work (burets, pipets, and volumetric flasks), while other glassware is not intended for such work (beakers, Erlenmeyer flasks, and graduated cylinders). In high accuracy/precision work, the glassware must be clean. Not only does clean glassware avoid unwanted chemical contamination, but it also assures that delivered volumes of liquids will be correct. A dirty spot on the inside wall of a burette or pipet, for example, even if the spot itself does not occupy a significant volume, can cause a droplet of water to adhere to the wall, causing an error in the recorded volume of delivered liquid (less volume delivered). Measuring and recording data. The generally accepted rule for measuring volumes is to estimate one more digit beyond the digit associated with the closest spaced markings. For measurements obtained from devices which provide digital output, such as electronic mass balances or a Spectronic 20, the digits should all be written, including any trailing zeroes, with the understanding that the last digit is within “one” unit of the correct value.

Errors (Uncertainties) in Measurements
Every measurement has a degree of uncertainty associated with it. The uncertainty derives from the measuring device and from the skill of the person doing the measuring. Let's use a volume measurement as an example. Say you are in a chemistry lab and need 7 mL of water. You could take an unmarked coffee cup and add water until you think you have about 7 milliliters. In this case, the majority of the measurement error is associated with the skill of the person doing the measuring. You could use a beaker, marked in 5 mL increments. With the beaker, you could easily obtain a volume between 5 and 10 mL, probably close to 7 mL, give or take 1 mL. If you used a pipette marked to with 0.1 mL, you could get a volume between 6.99 and 7.01 mL pretty reliably. It would be untrue to report that you measured...
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