Ch 6 Avogadro’s Number The first person to have calculated the number of molecules in any mass of substance was Josef Loschmidt (1821-1895) an Austrian high school teacher, who in 1865 calculated the number of molecules in one cubic centimeter of gaseous substance under ordinary conditions of temperature of pressure. The number was later changed to Avogadro’s number by Jean Perrin, a Noble Laureate, in a paper which won him the Nobel Prize.

Discovering NA The best modern values for what we now call "Avogadro's Number" are the result of the x-ray diffraction measurement of lattice distances in metals and salts. Today's best experimental value of   6.022 141 99 x 1023 atoms per mol is the best average for measurements using the best methods available.

Ch 6 The Mole The number represented by Avogadro’s constant is referred to as the mole (mol). It’s value for our calculations is 6.022e23 mol-1. The mole represents the number of atoms present in a mass equal to the atomic mass of an element. Therefore in 4.003 grams of hydrogen there are 6.022e23 atoms of helium. In 32.07 grams of Sulfur, there are 6.022e23 atoms of sulfur. The mole is a general representation used to represent something that is always equal to NA.

Ch 6 Molar Mass A mole represents not only a specific number of elemental particles but also can be used for compounds and formulas. The molar mass of a compound is numerically equal to the sum of the masses of atoms in the formula. The molar mass of H2O is 18.02 g/mol. Therefore, in 18.02 grams of water there are 6.022e23 molecules of water. Therefore, the mole (6.022e23) acts as a bridge between the mass of an atom or compound and the actual number of particles present.

Ch 6 Mole-Gram Conversions It is useful to use Avogadro's number and the mole to convert between atoms, moles, and grams of both product and reactant. The following formula converts between moles and grams of a substance. m = M * n mass = Molar mass times moles The number of moles is either given, or attained using NA.