Here are some useful physical constants, conversion factors, and unit prefixes. They are used in many calculations in chemistry, as well as in physics and other sciences.

## Useful Constants

A physical constant is also known as a universal constant or a fundamental constant. It is a quantity that has a constant value in nature. Some constants have units, while others do not. While the physical value of a constant does not depend on its units, obviously changing the units is associated with a numerical change. For example, the speed of light is a constant, but it is expressed as a different number in meters per second compared to miles per hour.

Acceleration of Gravity | 9.806 m/s
^{2} |

Avogadro's Number | 6.022 x 10
^{23} |

Electronic Charge | 1.602 x 10
^{-19} C |

Faraday Constant | 9.6485 x 10
^{4} J/V |

Gas Constant | 0.08206 L·atm/(mol·K)
8.314 J/(mol·K) 8.314 x 10 ^{7} g·cm
^{2}/(s
^{2}·mol·K) |

Planck's Constant | 6.626 x 10
^{-34} J·s |

Speed of Light | 2.998 x 10
^{8} m/s |

p | 3.14159 |

e | 2.718 |

ln x | 2.3026 log x |

2.3026 R | 19.14 J/(mol·K) |

2.3026 RT (at 25°C) | 5.708 kJ/mol |

## Common Conversion Factors

A conversion factor is a quantity used to convert between one unit and another via multiplication (or division). A conversion factor changes the units of a measurement without changing its value. The number of significant digits in a conversion factor may affect the conversion in some cases.

Quantity | SI Unit | Other Unit | Conversion Factor |
---|---|---|---|

Energy | joule | calorie
erg |
1 cal = 4.184 J
1 erg = 10 ^{-7} J |

Force | newton | dyne | 1 dyn = 10
^{-5} N |

Length | metre or meter | ångström | 1 Å = 10
^{-10} m = 10
^{-8} cm = 10
^{-1} nm |

Mass | kilogram | pound | 1 lb = 0.453592 kg |

Pressure | pascal | bar
atmosphere mm Hg lb/in ^{2} |
1 bar = 10
^{5} Pa
1 atm = 1.01325 x 10 ^{5} Pa
1 mm Hg = 133.322 Pa 1 lb/in ^{2} = 6894.8 Pa |

Temperature | kelvin | Celsius
Fahrenheit |
1°C = 1 K
1°F = 5/9 K |

Volume | cubic metre | litre
gallon (U.S.) gallon (U.K.) cubic inch |
1 L = 1 dm
^{3} = 10
^{-3} m
^{3}
1 gal (U.S.) = 3.7854 x 10 ^{-3} m
^{3}
1 gal (U.K.) = 4.5641 x 10 ^{-3} m
^{3}
1 in ^{3} = 1.6387 x 10
^{-6} m
^{3} |

While a student should learn how to perform unit conversions, in the modern world there are accurate online unit converters in all the search engines.

## SI Unit Prefixes

Metric system or SI units are based on factors of ten. However, most units prefixes with names are 1000 times apart. The exception are near the base unit (centi-, deci-, deca-, hecto-). Usually, a measurement is reported using a unit with one of these prefixes. It's a good idea to become comfortable converting between factors as they are used in all scientific disciplines.

Factors | Prefix | Symbol |
---|---|---|

10
^{24} |
yotta | Y |

10
^{21} |
zetta | Z |

10
^{18} |
exa | E |

10
^{15} |
peta | P |

10
^{12} |
tera | T |

19
^{9} |
giga | G |

10
^{6} |
mega | M |

10
^{3} |
kilo | k |

10
^{2} |
hecto | h |

10
^{1} |
deca | da |

10
^{-1} |
deci | d |

10
^{-2} |
centi | c |

10
^{-3} |
milli | m |

10
^{-6} |
micro | µ |

10
^{-9} |
nano | n |

10
^{-12} |
pico | p |

10
^{-15} |
femto | f |

10
^{-18} |
atto | a |

The ascending prefixes (e.g., tera, peta, exa) are derived from Greek prefixes. Within 1000 factors of a base unit, there are prefixes for each factor of 10. The exception is 10^{10}, which is used in distance measurements for the angstom..Beyond this, factors of 1000 are used. Very large or very small measurements are usually expressed using scientific notation.

A unit prefix is applied with the word for a unit, while its symbol is applied together with a unit's symbol. For example, it is correct to cite a value in units of either kilograms or kg, but it is incorrect to give the value as kilog or kgrams.

## Sources

- Cox, Arthur N., ed. (2000).
*Allen's Astrophysical Quantities*(4th ed.). New York: AIP Press / Springer. ISBN 0387987460. - Eddington, A.S. (1956). "The Constants of Nature". In J.R. Newman (ed.).
*The World of Mathematics*. 2. Simon & Schuster. pp. 1074–1093. - "International System of Units (SI): Prefixes for binary multiples." The NIST Reference on Constants, Units, and Uncertainty. National Institute of Science and Technology.
- Mohr, Peter J.; Taylor, Barry N.; Newell, David B. (2008). "CODATA Recommended Values of the Fundamental Physical Constants: 2006."
*Reviews of Modern Physics*. 80 (2): 633–730. *Standard for the Use of the International System of Units (SI): The Modern Metric System*IEEE/ASTM SI 10-1997. (1997). New York and West Conshohocken, PA: Institute of Electrical and Electronics Engineers and American Society for Testing and Materials. Tables A.1 through A.5.