1. Chapter 6
The mole

2. Measuring Matter What do you ask for when you buy: 2 shoes 12 eggs
48 doughnuts
500 sheets of paper
1 pair
1 dozen
4 dozen
1 ream

3. But the number they represent is always constant!
Pair, Dozen, Ream
These are all ways to batch a group of objects to make them easier to count!
The object may change
Eggs to Doughnuts
But the number they represent is always constant!

4. What do eggs have to do with Chemistry?
How many carbon atoms are in a teaspoon of carbon?
200,666,666,666,666,666,666,667 atoms!!!
Can you even pronounce this?

5. 200,666,666,666,666,666,666,667 atoms!!!
It would be nice if chemists had a batch like a dozen (but muchhhhhhh bigger) to handle this kind of number!

6. Meet the Mole!
He is the Chemist’s “dozen”!

7. What is a mole? The chemist’s “dozen” is called the:
MOLE (or the unit mol)
1 dozen atoms = 12 atoms
1 mole atoms = atoms Or
1 mole = 6.02  1023
How many moles are in 1 teaspoon of carbon atoms?
.33 moles

8. Measuring Matter – Moles and Avogadro’s number
Mole (mol) – the amount of a substance that contains the same number of particles as the number of atoms in 12 g of carbon-12. Like doughnuts are counted in dozens, the mole is a SI unit for counting the amount of a substance.
1 dozen pencils have the same number of units as 1 dozen doughnuts
1 mole of carbon atoms have the same number of particles as 1 mole of water molecules
This does not mean they both weigh the same, only that they have the same number of units or particles.

9. Measuring Matter – Moles and Avogadro’s number
Avogadro’s Number – the number of particles (6.02  1023) in exactly one mole of a pure substance.
1 mole pencils = 6.02  1023 pencils
1 mole water molecules = 6.02  1023 molecules of water

10. Measuring Matter – Moles and Avogadro’s number
If Avogadro’s number is the number of particles in 1 mole, how do you know what kind of particle you have?
Remember…
Particles can be an atom, molecule, or formula unit?

11. Particles can be an atom, molecule, or formula unit?
What kind of particle?
Particles can be an atom, molecule, or formula unit?
Atom – one atom
Molecule – more than one nonmetal atom
Formula unit - compound with a metal or NH4+

12. Measuring Matter – Moles and Avogadro’s number
How do you know what kind of particle you have?
Examples
KCl
N2O2 H2 Na
Formula unit
molecule
molecule
atom

13. Measuring Matter – Moles and Avogadro’s number
Avogadro’s number is really an equality!
1 mole C = 6.021023 atoms C
What can you use equalities to do?
Equalities are conversion factors in Dimensional Analysis problems!

14. Measuring Matter – Moles and Avogadro’s number
How many representative particles are in 3.2 mol of C?
6.021023
atoms C
= atoms C
1.91024 1
mol C

15. Measuring Matter – Moles and Avogadro’s number
How many moles of water molecules are in 3.7  1021 molecules of H2O? 1
mol H2O
3.7  1021 molecules H2O
= mol H2O
.0061
6.021023
Molecules H2O

16. How do you find this mass?
Mass and the Mole
Molar mass – the mass, in grams, of one mole of a substance.
How do you find this mass?
The atomic mass printed on the periodic table has two meanings – it is the average mass of an atom in atomic mass units (u) and the mass of one mole of atoms of a given element in grams.

17. Mass and the Mole Finding molar mass 1mole H atoms = 1mole C =
Atoms – the mass of 1 mole of any atom is the same as the atomic mass in grams.
1mole H atoms = g
1mole C = g
1mol Cu = g

18. Mass and the Mole Molar mass continued 1mole H2O = 18.0153g 2 H =
Compounds – the mass of 1 mole of a compound is the sum of the masses of the atoms.
1mole H2O = g
2 H =
2( g) = g
1 O = g
g g = g

19. Practice!!
Find the molar mass of the following! CCl4 CaO (NH4)2SO4

20. Mass and the Mole Molar mass is really an equality! For water:
1mole H2O = g
What can you use equalities to do?
Equalities are conversion factors in Dimensional Analysis problems!

21. Mass and the Mole How many moles are in 242 g of water? = mol H2O
242 g H2O 1
mol H2O
= mol H2O
13.4
g H2O

22. Mass and the Mole What is the mass of 3.77 mol of Au? = g Au
3.77 mol Au
g Au
= g Au
743 1
mol Au

23. Multi Step Problems What is the mass of 5.28 x 1028 molecules of water? g =
1.58x106 g
6.02x1023 1
molecules
mole

24. The Last Conversion Factor!
What do you do if you are asked to find atoms and your representative particle is NOT an atom?

25. The Formula Conversion Factor
Think about what a formula really tells you!
1 molecule of H2O
Doesn’t this mean:
1 molecule H2O = 2 hydrogen atoms
And ….
In 1 mole of H2O there are 2 moles of Hydrogen
The formula is a conversion factor between atoms and compounds!

26. The Formula Conversion Factor
How many hydrogen atoms are in 234 molecules of water?
234 molecules H2O 2
atoms H
= atoms H
468 1
molecules H2O

27. How many Hydrogen atoms are in 54.2g of H2O?1 2
mol of H =
mol
6.02x1023
Atoms
3.62x1024
atoms 1 1
mol of H
mol of H2O g

28. Conversion Review Mass Particles Moles Avogadro’s # Molar mass Formula
Atoms

29. Percent Composition
How would you calculate the percent females in this room?
All percents are calculated in the same way!

30. Percent Composition
Percent Composition – the percent by mass of each element in a compound.
molar mass

31. Percent Composition
What is the percent composition of each element in water?
Mass H =
Mass O =
2( g) = g + g g
% H = %
% O = %

32. Using Percent Composition
If a glass of water contains 648 g of water, how many grams of hydrogen would it hold?
Remember, water is % hydrogen
What is % of 648 g? g
72.5 g

33. C3H8O
Isopropyl Alcohol

34. Find the % composition of the elements in rubbing alcohol
Molar Mass = C? H? O? g % % %

35. Empirical Formula What does H2O mean?
Does it mean 2 atoms of H for every atom of O?
YES
Does it mean 2 g of H for every 1 g of O?
NEVER

36. Empirical Formula What does H2O mean?
Does it mean 2 moles of H for every mole of O?
Always!
Formulas are not only ratios of atoms, they are also ratios of MOLES

37. Formulas are ratios of moles!

38. Circle the empirical formulas!
Empirical Formula: simplest whole number ratio of moles of the atoms in a substance. Experimental method that is the first step in finding the formula of a compound.
Circle the empirical formulas!
H2O
NaCl
C2H6
NO2
C6H12O6
H2SO4
N2O4

39. Finding the empirical formula
Find the mass of each element in the compound.
Usually given
If given as %, then change % to g.
25% H and 75% C  25 g H and 75 g C
Convert masses to moles.
Use molar masses.
25 g H 1
mol H
= mol H 25
g H

40. Empirical Formula Find the smallest whole number ratio of moles.
a. Write the results of step 2 like a formula.
If C = 6.2 mol and H = 25 mol
C6.2H25
b. Divide by the smallest mole amount.
c. If not all whole numbers, multiply by 2,3, or 4 …
CH4
6.2

41. Empirical Formula Examples for step 3 X3Y4 X3Y2 X.029Y.039 X.009Y.006
X = .029 mol X = .009 mol
Y = .039 mol Y = .006 mol
X.029Y.039
X.009Y.006
X1Y1.34
X1.5Y1
Multiply by 3
Multiply by 2
X3Y4
X3Y2

42. Empirical Formula More Examples for step 3 X1Y3 X2Y5 X1.47Y3.68 X1Y2.5
X = 1.47 mol X = 2.4  10-4 mol
Y = 3.68 mol Y = 7.3  10-4 mol
X1.47Y3.68
2.4 10-4
X1Y2.5
X1Y3.04
Multiply by 2
X1Y3
X2Y5

43. Empirical Formula - Example
A sample of an unknown gas contains 43.2 g of carbon and g of oxygen. What is the empirical formula?
Find Masses
43.2 g C g O
Change to moles
43.2 g C 1
mol C
= mol C
3.60
12.0
g C
115.8 g O 1
mol O
= mol O
7.24
16.0
g O

44. Empirical Formula - Example
Get whole numbers
C3.60O7.24
C1O2.01
CO2

45. Empirical Formula
Find the empirical Formula of 52.8% Sn, 12.4% Fe, 16.0%C, & 18.8% N.

46. Molecular Formula
Molecular formula - is some whole number multiple of the empirical formula.
HO is an empirical formula
H2O2 is twice HO
(HO)X and X = 2
For C6H12O6, the empirical formula is CH2O and X=6
To convert an empirical formula to a molecular formula you must find X.

47. Molecular Formula - Example
An unknown gas is found to have an empirical formula of NO2 and a molar mass of g/mol. What is the molecular formula?
Molecular formula = (NO2)X
= (NO2)2
= N2O4