The Mole Mrs. Coyle Honors Chem

-How do we measure chemical quantities -How do we measure chemical quantities? -What units of measure do we use?

Atomic Mass Unit (amu): = 1/12 of the mass of a 12C atom

Relationship of amu to gram 1 amu = 1.66054 x 10 -24 g

Atomic Mass: The average of the masses of the naturally occurring isotopes of an element weighted according to their abundances.

Atomic Mass of 12C: 12 amu= 1.992648x10-23g

Why the Mole? Are the quantities scientists use on a lab scale at the level of the atom or at the level of grams?

The Solution Why not use a unit measure that uses the quantities equal to the atomic masses of the elements, but measured (expressed) in grams.

One mole of atoms of an element is contained in an amount of the element equal to its atomic mass measured in grams. This mass is called molar mass.

Molar Mass: The mass in grams of one mole of a substance. For an element, it is the atomic mass expressed in grams. Example: for Neon, Atomic Mass = 20.18 amu Molar Mass = 20.18 g Other names used traditionally for molar mass are “molecular weight” and “gram formula mass”.

The Definition of The Mole The mass of an element or compound that contains as many elementary particles (atoms, molecules, ions, etc.) as there are atoms in exactly 12 grams of 12C.

How many atoms are there in 12 g of C-12. 6 How many atoms are there in 12 g of C-12 ? 6.022145 x 1023 atoms 1 mole of atoms (mol) Avogadro’s Number

Avogadro’s Number Can be used the same way the word “dozen” is used to mean 12 for any object. One mole of books is 6.022x1023 books, just like a dozen books is 12 books.

Question How can 12g of C-12 have the same # of atoms as 1.008g of H?:

Solution: 1 atom of Hydrogen has an atomic mass of 1.008 amu. 1 atom of Carbon-12 has an atomic mass of 12 amu. 1 atom C-12 = 12 amu = 12 1 atom H 1.008 amu 1.008 6.022x1023 atoms C-12 = 12 g = 12 6.022x1023 atoms H 1.008 g 1.008

Relationship of amu to gram revisited: For C-12 (6.022x1023 atoms)(12amu)=12 g atom 6.022x1023amu= 1 g

Problem 1 What is the mass in grams of 7 atoms of Neon? Solution: (7 atoms)( 20.18 amu/atom)= 141.26 amu 141.26 amu x 1 g = 2.35x10-22g 6.022x1023amu

“Atom” vs “Mole of Atoms of Hydrogen” Atomic Mass Molar Mass Number of Atoms 1 atom 1.008 amu= 1.67382432x10-24 g 1 1 mole 1.008 g 6.022 x 1023

“Molecule” vs “Mole of Molecules of Hydrogen” Molecular Mass Molar Mass Number of Molecules 1 molecule 2.016 amu 1 mole 2.016 g 6.022 x 1023

Mathematical Relationships #mol= Mass of Sample (g) Molar Mass (g/mol) #particles=#mol x 6.022 x10 23 particles/mol 1 g = 6.022x1023amu

Problem # 2 For a sample of 20.0g of iron, Fe, calculate: a) the number of moles of atoms b) the number of atoms Solution: #mol Fe= mass Fe(g) = 20g Fe molar mass Fe(g/mol) 55.85g/mol #mol Fe = 0.358 mol Fe atoms

Problem # 2 Cont’d b) #atoms Fe= #mol x (6.022 x 1023 atoms/mol)= = (0.358 mol Fe)(6.022 x 10 23 atoms/mol) #atoms Fe= 2.156x1023 atoms Fe

Problem # 3 For a sample of 150.0 g of H2SO4, calculate: The number of moles of molecules of H2SO4 present. The number of oxygen atoms present. Solution: MM H2SO4 = H: (2 atoms)(1.008g/mol) S: (1 atom)(32.07g/mol) O: (4 atoms)(16.00g/mol) 98.09g/mol

Problem # 3 # mol H2SO4 = Mass (g) =150.0g H2SO4 MM (g/mol) 98.09g/mol # mol H2SO4= 1.529 mol H2SO4 molecules

Problem # 3 Cont’d b) #atoms of O = (#mol O)(6.022x1023 atoms)= = (4 x 1.529 mol O) (6.022x1023 atoms) = = 3.683 x1024 atoms O

Molar Volume of an Ideal Gas At STP conditions, Standard Temp of 0 deg C and Standard Pressure of 1 atm, the molar volume of an ideal gas is 22.4 L

For an ideal gas at STP: #mol= Volume (L) 22.4 (L/mol)

Problem # 4 #mol O2= Volume (L) = 44.8 L = 2.00 mol O2 How many atoms of oxygen are there in 44.8L of oxygen gas? Solution: #mol O2= Volume (L) = 44.8 L = 2.00 mol O2 22.4 (L/mol) 22.4 L/mol #atoms O =#mol x 6.022 x10 23 atoms/mol = 2 x (2.00mol O)(6.022 x10 23 atoms/mol) #atoms O= 2.41 x 1024 atoms O