Measurement is the collection of quantitative data. The proper handling and interpretation of measurements are essential in chemistry - and in any scientific endeavour. To use measurements correctly, you must recognize that measurements are not numbers. They always contain a unit and some inherent error. The second lecture focuses on an international system of units (the SI system) and introduces unit conversion. In the third lecture, we'll discuss ways to recognize, estimate and report the errors that are always present in measurements.
Quantity | SI Base Unit |
English Equivalent |
length | meter (m) | 1 m = 39.36 in |
mass | kilogram (kg) | 1 kg = 2.2 lbs |
time | second (s) | |
temperature | kelvin (K) | °F = 1.8(^{o}C)+32 K = °C + 273.15 |
Quantity | Dimensions | SI units | Common name |
area | length × length | m^{2} | square meter |
velocity | length/time | m/s | |
density | mass/volume | kg/m^{3} | |
frequency | cycles/time | s^{-1} | hertz (Hz) |
acceleration | velocity/time | m/s^{2} | |
force | mass × acceleration | kg m/s^{2} | Newton (N) |
work, energy, heat | force × distance | kg m^{2}/s^{2} | Joule (J) |
Prefix | Meaning | Abbreviation | Exponential Notation |
Giga- | billion | G | 10^{9} |
Mega- | million | M | 10^{6} |
kilo- | thousand | k | 10^{3} |
centi- | hundredths of | c | 10^{-2} |
milli- | thousandths of | m | 10^{-3} |
micro- | millionths of | µ | 10^{-6} |
nano- | billionths of | n | 10^{-9} |
pico- | trillionths of | p | 10^{-12} |
Non SI unit | Unit type | SI conversion | Notes |
liter (L) | volume | 1 L = 1000 cm^{3} | 1 quart = 0.946 L |
Angstrom (Å) | length | 1 Å = 10^{-10} m | typical radius of an atom |
atomic mass unit (u) | mass | 1 u = 1.66054×10^{-27} kg | about the mass of a proton or neutron; also known as a 'dalton' or 'amu' |
good precision & good accuracy poor accuracy but good precision |
good accuracy but poor precision poor precision & poor accuracy |
Precision | Accuracy |
reproducibility | correctness |
check by repeating measurements | check by using a different method |
poor precision results from poor technique | poor accuracy results from procedural or equipment flaws |
poor precision is associated with 'random errors' - error has random sign and varying magnitude. Small errors more likely than large errors. | poor accuracy is associated with 'systematic errors' - error has a reproducible sign and magnitude. |
Score by distance from bullseye | |
Score by area or target |
Copyright © 1997-2005 by Fred Senese
Comments & questions to fsenese@frostburg.edu
Last Revised 06/16/05.URL: http://antoine.frostburg.edu/chem/senese/101/measurement/print-index.shtml