Avogadros Number The Mole

Avogadro s number (N) is defined as the number of carbon-12 atoms in exactly 12 g of carbon 12. Its value is approximately 0.60229 X 10-. It was named for the Italian physicist Amedeo Avogadro, whose work is discussed in Section 4-10. 

A mole of a substance is defined as Avogadro s number of molecules of the substance. Thus a mole of water, H O, is the quantity of water containing N H2O molecules. The molecular mass of water (the sum of the atomic masses of the atoms in the molecule 2 x 1.00797 -f- 1 x 15.9994, from Table 4-1) is 18.0153. From the definitions of atomic masses and the mole we see that a mole of water is 18.0153 g. 

A mole of iodine atoms is 126.9044 g of iodine, and a mole of iodine molecules (I.,) is 253.8088 g of iodine. Sometimes, to prevent confusion, a mole of atoms of an element is called a gram-atom. The mass of a mole of a compound for which a formula is written is sometimes taken to be the gram-formula-mass (gfm), the number of grams equal to the sum of the atomic masses corresponding to the formula, whether or not the formula is a correct molecular formula for the substance. Thus the molar mass of liquid acetic acid, for which the formula CH3COOH(l) is written, is taken 

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