Skoog Chapter 5 Flashcards
Explain the difference between random & systematic errors.
Random error causes data to be scattered more or less symmetrically around a mean value while systematic error causes the mean of a data set to differ from the accepted value.
Suggest two sources of systematic error and two sources of random error in measuring the width of a 3-m table with a 1-m metal rule.
(1) Meter stick slightly longer or shorter than 1.0 m – systematic error.
(2) Markings on the meter stick always read from a given angle – systematic error.
(3) Variability in the sequential movement of the 1-m metal rule to measure the full 3-m table width – random error.
Describe at least three systematic errors that might occur while weighing a solid on an analytical balance.
(1) The analytical balance is miscalibrated.
(2) After weighing an empty vial, fingerprints are placed on the vial while adding sample to the vial.
(3) A hygroscopic sample absorbs water from the atmosphere while placing it in a weighing vial.
Describe at least three ways in which a systematic error might occur while using a pipet to transfer a known volume of liquid.
(1) The pipet is miscalibrated and holds a slightly different volume of liquid than the indicated volume.
(2) The user repetitively reads the volume marking on the pipet from an angle rather than at eye level.
(3) The inner surfaces of the pipet are contaminated.
How are systematic method errors detected?
- Using known standards
- Comparing methods
- Blank Tests
- Instrument Calibration
- Repeat measurements
What kind of systematic errors are detected by varying the sample size?
Both constant and proportional systematic errors can be detected by varying the sample size. Constant errors do not change with the sample size while proportional errors increase or decrease with increases or decreases in the samples size.
What is the standard error of a mean? Why is the standard deviation of the mean lower than the standard deviation of the data points in a set?
The standard deviation describes the variability within a single sample.
The standard error of the mean describes the variability of sample means.
