Specifying the amount in moles is useful in chemistry becuase the number of each type of molecule in a reaction is generally extremely important. This unit gives an idea of the precision exercised in measuring out- it is a reference to the exact number of molecules of a given substance.
1 mole is a very large, fixed number of molecules of any substance- in fact the exact number, called Avagadro's constant, is 6.0228x10
23.
To work out how much a mole of something weighs, then, consult your friendly periodic table for the masses of the atoms comprising the molecule in question, and add up all the masses of all the atoms in your molecule... and voila, that number is the Molecular Weight of that molecule, and the mass in grams of 1 mole of molecules of your substance.
The Molecular Weight is usually written on the label of lab reagents, and you may also find the M.W. of your substance in many online chemical databases.
For example, one mole of carbon weighs 12.011 grams, a mole of water weighs 18 g and
tetrabutylammonium iodide,
formula
C
16H
36IN has
Molecular Weight 12x16+1x36+126.9x1+14x1 = 368.9
Therefore 1 mole, or 6.0228x10
23 molecules of TBAI, weighs 368.9 g.
Correction: Avagadro's constant = 6.0228 x 10
23