The Mole

1. The Mole A mole is the number of particles equal to the number of Carbon atoms in 12 g of C-12 One mole = 6.0221417930 x 1023 units The number of particles in 1 mole is called Avogadro’s Number This value was not determined by a single experiment nor a single individual. The first steps to its determination were accomplished by Johann Josef Lohschmidt an Austrian physicist working in Vienna who, in 1865, using Maxwell’s equation for sphere diameter (developed from the Kinetic – Molecular Theory) estimated the average diameter of the molecules in air. He did this by estimating the number of particles in a given volume of gas. This value, the number density of particles in an ideal gas, is often referred to as Lohschmidt’s number.

2. In 1909 Jean Perrin combined Lohschmidt’s work and Avogadro’s Principle (want me to sing it for you?) to calculate a value for N for Hg of 6.2 x 1023. He also proposed that the value for the constant be named in honor of Amedeo Avogadro. Today we use laser crystallography to determine the number of particles in a certain sample of a substance and thus have a very precise and accurate value for NA. Remember, this is the magic number that allows us to turn atomic masses (amu) into grams. • We define the masses of atoms, molecules, and formula units in terms of atomic mass units. • 1 Carbon atom = 12.01 amu • 1 O2 molecule = 2(16.00 amu) = 32.00 amu • 1 MgCl2 F.U. = 24.31 + 2(35.45) = 95.21 amu Atomic mass(Ar), molecular mass(Mr), and formula mass are ALL measured in amu

3. The molar mass is the mass in grams of one mole of a compound C = 12.01 amu = 12.01 g O2 = 32.00 amu = 32.00 g MgCl2 = 95.21 amu = 95.21 g Since we cannot count individual atoms and molecules, the mole allows us to count by weighing. To do this we must be able to convert between moles and mass. A .267 mol sample of magnesium nitride equals how many grams? A 15.2 g sample of sulfuric acid would contain how many moles of molecules? Hydrogen atoms? How many molecules are in a swallow of water? How many carbon atoms are in a sugar cube? Now try it in reverse.