Ch 6 Avogadro’s Ch 6 Avogadro’s NumberNumber

The first person to have calculated the The first person to have calculated the number of molecules in any mass of number of molecules in any mass of substance was Josef Loschmidt (1821-substance was Josef Loschmidt (1821-1895) an Austrian high school teacher, 1895) an Austrian high school teacher, who in 1865 calculated the number of who in 1865 calculated the number of molecules in one cubic centimeter of molecules in one cubic centimeter of gaseous substance under ordinary gaseous substance under ordinary conditions of temperature of pressure.conditions of temperature of pressure.

The number was later changed to The number was later changed to Avogadro’s number by Jean Perrin, a Avogadro’s number by Jean Perrin, a Noble Laureate, in a paper which won Noble Laureate, in a paper which won him the Nobel Prize.him the Nobel Prize.

Discovering NDiscovering NAA

The best modern values for what we The best modern values for what we now call "Avogadro's Number" are now call "Avogadro's Number" are the result of the x-ray diffraction the result of the x-ray diffraction measurement of lattice distances in measurement of lattice distances in metals and salts. metals and salts.

Today's best experimental value of   Today's best experimental value of   6.022 141 99 x 106.022 141 99 x 102323 atoms per mol is atoms per mol is the best average for measurements the best average for measurements using the best methods available. using the best methods available.

Ch 6 The Ch 6 The MoleMole

The number represented by Avogadro’s The number represented by Avogadro’s constant is referred to as the mole (mol).constant is referred to as the mole (mol).

It’s value for our calculations is 6.022e23 molIt’s value for our calculations is 6.022e23 mol -1-1.. The mole represents the number of atoms The mole represents the number of atoms

present in a mass equal to the atomic mass of present in a mass equal to the atomic mass of an element.an element.

Therefore in 4.003 grams of helium there are Therefore in 4.003 grams of helium there are 6.022e23 atoms of helium. In 32.07 grams of 6.022e23 atoms of helium. In 32.07 grams of Sulfur, there are 6.022e23 atoms of sulfur.Sulfur, there are 6.022e23 atoms of sulfur.

The mole is a general representation used to The mole is a general representation used to represent something that is always equal to Nrepresent something that is always equal to NAA..

Ch 6 Molar MassCh 6 Molar Mass A mole represents not only a specific A mole represents not only a specific

number of elemental particles but also can number of elemental particles but also can be used for compounds and formulas.be used for compounds and formulas.

The molar mass of a compound is The molar mass of a compound is numerically equal to the sum of the numerically equal to the sum of the masses of atoms in the formula.masses of atoms in the formula.

The molar mass of HThe molar mass of H22O is 18.02 g/mol. O is 18.02 g/mol. Therefore, in 18.02 grams of water there Therefore, in 18.02 grams of water there are 6.022e23 molecules of water.are 6.022e23 molecules of water.

Therefore, the mole (6.022e23) acts as a Therefore, the mole (6.022e23) acts as a bridge between the mass of an atom or bridge between the mass of an atom or compound and the actual number of compound and the actual number of particles present.particles present.

Ch 6 Mole-Gram Ch 6 Mole-Gram ConversionsConversions

It is useful to use Avogadro's number It is useful to use Avogadro's number and the mole to convert between and the mole to convert between atoms, moles, and grams of both atoms, moles, and grams of both product and reactant.product and reactant.