1. The Mole
Mrs. Coyle Honors Chem

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

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

4. Relationship of amu to gram
1 amu = x g

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

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

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

8. 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.

9. 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.

10. 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 = amu
Molar Mass = g
Other names used traditionally for molar mass are “molecular weight” and “gram formula mass”.

11. 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.

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

13. 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.

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

15. 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 amu
6.022x1023 atoms C-12 = 12 g = 12
6.022x1023 atoms H g

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

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

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

19. “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

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

21. 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 = mol Fe atoms

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

23. 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

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

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

26. 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

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

28. 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) L/mol
#atoms O =#mol x x10 23 atoms/mol
= 2 x (2.00mol O)(6.022 x10 23 atoms/mol)
#atoms O= 2.41 x 1024 atoms O