Astrophysics (Index)About


(unit roughly the count of hydrogen atoms in a gram)

A mole of a pure compound (all the same molecule species) is a number of grams that (roughly) matches the number of nucleons (protons and neutrons) in the molecule, i.e., a mole of atomic hydrogen (having a nucleus of one proton) is roughly one gram, of atomic helium is roughly 4 grams, etc. The SI standard mole is a fixed number of particles, the number termed Avogadro's number, currently defined to be 6.02214076 × 1023 particles. The unit is convenient for chemists, who can relate grams to elements and molecular formulae, and can be convenient in calculations for allowing smaller numbers to be used than for particle counts. Earlier in its history, Avogadro's number and the mole were pinned to the standard gram along with some selected element isotope, but (like astronomical unit) it is now considered more useful to have a stable, precise number not subject to future change due to improving measurement.

Further reading:

Referenced by pages:
Avogadro's number (NA)
ideal gas law
mean molecular weight (μ)
mixing ratio
acidity (pH)