The Mole

Matter on a human scale is commonly measured by weighing or by counting.  You wouldn't want to count the number of pieces of coal in a trainload of coal but you might find it more convenient to count the number of cars used to haul  the coal.  The mole is a counting unit.  It represents a fixed number of units of anything just like a dozen pairs of socks, or a score of elephants has the same meaning regardless of the objects being counted.

The SI definition of the mole is : "the number of objects equal to the number of atoms in exactly 12 g of carbon-12". This number is 6.023 x 1023 and is known as:              Avagadro's number( or NA).  Since the mole is just a counting unit,  1 mole of carbon-12 contains  6.023 x 1023 atoms, 1 mole of water also contains 6.023 x 1023 molecules of water and one  mole of electrons contains 6.023 x 1023 of electrons.

Since each element has a fixed total mass, one mole of each of the elements also has a fixed total mass. This property means that you can count atoms by weighing any amount of an element, and you can accurately predict the mass of any number of atoms if you know what element you have.  Here's how.  Using a periodic table, or a listing of elemental atomic masses, you can obtain the atomic mass (also atomic weight) of an element.  For example, you can find the mass of one mole, or 6.023 x 10²³ atoms of sodium as 22.99 grams. The mass of a mole of lead atoms is much heavier at 207.2 grams of lead.
 
 
 
 
 

The mass of a mole of molecules is just the sum of the masses of all of its elements, so that the mass of one mole of water, H₂O , can be calculated as:
 

2 atoms of H x 1.004 grams/mole of H + 1 atom of O 16.00 grams/mole of O = 18.02 g

If you would like more practice with molar conversions, visit this website. You will find an interactive periodic table that converts amounts of elements between moles and mass as well as other conversions.