Mole and Avogadro's number

Chemists usually deal with macroscopic quantities of various elements and compounds. It is therefore of particular interest to relate the actual number of atoms in an amount of an element to its mass in grams. For example, how many atoms are present in 12.01 grams of carbon, in 1.008 grams of hydrogen, or in 32.04 grams of sulfur? We know, of course, that the number of atoms in each of these quantities is the same. But what is the number? This number has been determined experimentally in a number of different ways and turns out to be an extremely large number-- 6.022 x 1023. It is known as Avogadro's number (NA), in honor of the Italian chemist.

Sample problem: How many atoms are present in 24 g of carbon?
Solution: 24 g of carbon is 2 moles or 1.2 x 1024.

Sample problem: What mass of silver contains the same number of atoms as 12 g of carbon?
Solution: Since 12 g of carbon is one mole, we need one mole of silver or 107 g.

Atomic mass and weight

Molecular weight

 

Periodic table


Return to class notes TOC.

Page last modified: