SCIENCE - SI BASE UNITS
Quantity
Amount of substance
Unit
mole
Symbol
mol
Definition
The mole is the amount of substance of a system which contains as many elementary entities as there are atoms in 0.012 kilograms of carbon-12; its symbol is "mol". When the mole is used, the elementary entities must be specified and may be atoms, molecules, ions, electrons, other particles, or specified groups of such particles. In this definition, it is understood that unbound atoms of carbon-12, at rest and in their ground state, are referred to.
Comments
This definition was adopted in 1971, and the clarifying final sentence added in 1980. Prior to this, the mole and atomic weights in general were referred to the mass of oxygen-16.
It is, of course, extremely difficult to measure the mass of any atom, even carbon-12, accurately. However there is a well-used method of measuring the ratios of the masses of two atoms, called Penning trap mass spectrometry. Using carbon-12 as one of these atoms, the mass, and hence the number of moles, of the other atom can be calculated to high accuracy.
Other methods of measuring the amount of substance include:
i) 1 mole of any ideal gas always occupies the same volume at a given temperature and pressure; it is simple to measure the ratios of the amounts of any two gas samples. Also the corrections for non-ideal gas behaviour are well known.
ii) Electrolysis provides a quantitative method of measuring the amount of substance by measuring the amount of electricity. For example 1 mol of silver will be deposited with the same amount of electricity as 0.5 mol of copper.
iii) Using the laws of extremely dilute substances.
References
The International System of Units
, Bureau International des Poids et Mesures, 7
th
edition, 1998
[
H O M E
] [
S C I E N C E
] [
M A T H S
] [
A S T R O N O M Y
]
Download
|
Contact
|
About
Developed By
DOMEX E-DATA PVT. LTD.,
