Course Outline

General Chemistry

04/25/09

A mole is a quantity of matter that has the same mass in grams as its atomic or formula weight.

Atomic weight—the average mass of the atoms of an element taking into account the masses of the various isotopes and their relative abundance. For example, oxygen has an atomic weight of 15.9994. This means that the average of the atoms’ masses is 15.9994 atomic mass units (amu).  

Avogadro’s Number and the Mole

A mole is a collection of atoms with a mass equal to the atomic weight in grams. The number of atoms in a mole is 6.02 x 10²³. This works because one amu is 1.66 x 10^-24 gram, so it takes 6.02 x 10²³ of them to make one gram. This is one of the most useful concepts in chemistry. The atomic weight of lithium is 6.941amu. This means that a sample with a mass of 6.941 grams has 6.02 x 10²³ atoms of lithium in it. Note that this works for molecules and ions, too. The number 6.02 x 10²³ is known as Avogadro’s number in honor of the chemist who first proposed a law relating the volume of a gas to the number of gaseous particles it contains. Note that the law works for mass, too, but it was easier for chemists in the 17^th century to measure the volume of a gas than to weigh it.

Concept Check: How many atoms of tin are in a sample with a mass of 9.74 grams?

Answer: Use unit analysis and treat the atomic weight and Avogadro’s number as conversion factors.

Atomic mass and abundance data were found at Web Elements (September 16, 2004).

© Copyright 2006, Kelley Whitley, ChemProfessor. All rights reserved.

This site was last updated 08/04/06