1. Atomic Masses & the MOLE…

2. Neon is a gas that can be found in the atmosphere.
Dry air contains about 0.002% Ne
That’s 500,000,000,000,000,000 atoms of neon every breath
Scientists can deal with atoms quant-itatively because they know some fundamental properties of the atoms of each element

3. An atom of Oxygen -16 literally weighs 0.0000000000000000000000266 g
Masses of atoms expressed in grams are extremely small.
An atom of Oxygen -16
literally weighs g

4. -For most calcs in chemistry it is easier to use a relative atomic mass.
- For relative masses, an atom was arbitrarily chosen as the standard, by which all other masses are compared

5. Atomic Mass
A. Relative Mass is when you measure the mass of all atoms based on the measurement of one particular atom.

6. 6
CARBON C 12
C-12 atom acts as the standard to compare all other atoms
A single C-12 atom was assigned a mass of 12 atomic mass units (amu)
1 amu is exactly 1/12 of the mass of a C-12 atom
Carbon has 12 nucleons therefore, the mass of a proton = 1 amu

7. 3. The masses of all other atoms are based on the mass of 1 amu
H has 1 proton= 1 amu
He has 2 protons and nuetrons= 4 amus

8. A proton weighs amu
A neutron weighs amu

9. A. Most elements exist in nature as isotopes
II. Average Atomic Mass
A. Most elements exist in nature as isotopes
75% of all of the Cl atoms found in nature are the isotope Cl-35
25% of the all of the Cl atoms found in nature are the isotope Cl-37

10. 1.Average atomic mass is the mass on the PT and is the average of all of the naturally occurring isotopes of an element

11. Example:
We have 25 marbles that each weigh 2 g and 75 marbles that weigh one gram. What is the average atomic mass for these marbles?

12. 2. Calculating average atomic masses:
a. Cu
69.17% Cu-63 has a mass of 63 amu, 30.83% Cu-65 has a mass of 65 amu.
It’s calculated by multiplying the atomic mass of each isotope by its abundance and adding the results.

13. 63.6 amu Cu-63 (0.6917)(63 amu) Cu-65 + (0.3083)(65 amu)
The reported average atomic mass of naturally occurring Cu is amu.

14. Your Turn: b. Practice #1. Gallium consists of two isotopes of masses
68.95 amu and amu with abundances of 60.16% and 39.84%, respectively. What is the average atomic mass of gallium?

15. c. Practice #2.
Naturally occurring element X exists in three isotopic forms:
X-28 ( amu, 92.21% abundance),
X-29 ( amu, 4.70% abundance),
X-30 ( amu, 3.09% abundance)
. Calculate the atomic weight of X.

16. B. Relating Mass to the number of Atoms

17. It is impractical to get caught up into working with 1 atom’s mass because usually you working with a large number of atoms.
Scientists were challenged to figure out a way to define a collection of particles
And which could still be described in terms of a relative mass

18. 1. The average C atom with an atomic mass of 12
1. The average C atom with an atomic mass of 12.0 amus is 12 times heavier than the average H atom
So 100 C atoms are 12 times heavier than 100 H atoms
b. any # of C atoms would be 12 times heavier than the same # of H atom

19. If we assume we can relate the relative masses from the PT in the terms of grams
With the literal masses of an atom (g) for each atom we use we seem to get the same number of atoms
Oxygen:

20. Counting Atoms
Hydrogen:
Carbon:

21. Counting Atoms
The number of atoms that are in the relative masses of elements identical in grams to the atomic masses on the PT is called Avogadro’s Number
And is defined as a mole
The mole becomes a counting unit much like the dozen.
1 dozen eggs = 12 eggs
1 mole of eggs =
602,000,000,000,000,000,000,000 eggs

22. Counting Atoms
The word “mole” was introduced by Wilhelm Oswald, who derived the term from the word moles meaning a “heap” or “pile.”
We are just defining the pile as con-taining 6.02x1023 items
Because it is such a huge number of items we usually reserve the mole for atoms or molecules
1 mole is always = x 1023 items

23. Counting Atoms
The mole, whose abbreviation is “mol”, is the SI base unit for meas-uring amount of a pure substance.
The mole is the chemist’s six-pack or dozen. Many objects in our everyday lives come in similar counting units.
1 ream = 500 sheets of paper
1 pair of socks = 2 socks
1 cube = 24 cans

24. REPRESENTATIVE PARTICLES & MOLES
Atomic Nitrogen
Atom N
6.02x1023
Nitrogen gas
Molec. N2
Water
H2O
Calcium ion
Ion
Ca2+
Calcium Fluoride
Formula unit
CaF2
CHEMICAL
REP. PARTICLE
SUBTANCE
REPRESENTATIVE
FORMULA
PARTICLE
IN 1 MOL

25. Counting Atoms
We will learn in the near future that when a chemist reasons through a chemical reaction (recipe) they think in terms of X number of mols of this compound reacts with Y number of mols of this compound.
So we need to know how to calculate the number of molecules or atoms are involved in that X number of mols

26. How many moles of Magnesium
Counting Atoms
Using the mole in calculations #1
How many moles of Magnesium
is 1.25 x 1023 atoms of Magnesium?
unit equality:
1 mol Mg = 6.02 x 1023 atoms Mg

27. Counting Atoms = .208 mol Mg The desired conversion is: atoms moles
1 mole Mg
1.25x1023 atoms Mg
6.02x1023atoms Mg
= .208 mol Mg

28. Your Turn:
To make sure adequate amounts of product AB is being made during a rxn you might need mols of A reacting with 2.68 mols of B. How many atoms of A are reacting with how many atoms of B?
If you burn sugar (C12H22O11) in pure oxygen you produce carbon dioxide and water as products. To burn 6.02x1023 molecules of sugar you also need 7.22x1024 molecules of oxygen and you produce 6.62x1024 molecules of water and 7.22x1024 molecules of carbon dioxide. How many mols are reacted or produced for each component of the rxn?

29. Counting Atoms
Now suppose you want to determine how many atoms are in a mol of a compound or molecule
To do this you must know how many atoms are involved in the molecule.
To determine the number of atoms represented in a molecule requires knowing the chemical formula
Eg, each molecule of Carbon Dioxide (CO2) is composed of 3 atoms

30. Counting Atoms
1 mole of Carbon Dioxide contains Avogadro’s number of Carbon Dioxide molecules.
Thus a mole of CO2 contains three times Avogadro’s # of atoms

31. How many atoms of Carbon are in 2.12 mols of Propane (C3H8)?
Counting Atoms
Using the mole in calculations #2
How many atoms of Carbon are in 2.12 mols of Propane (C3H8)?
unit equalities:
1 mol C3H8 = 6.02x1023 molecules C3H8
1 molecule C3H8 = 3 atoms C

32. The desired conversions are:
Counting Atoms
The desired conversions are:
moles  molecules  atoms
6.02x1023 molecules C3H8
2.12
mols C3H8
3 atoms C
1 molecule C3H8
1 mole C3H8
=3.83x1024 atoms C

33. Your Turn:
Back to burning sugar, 1 mol of sugar (C12H22O11) was burned with 12 mols of oxygen (O2) to produce 11 mols of water (H2O) and 12 mols of Carbon dioxide (CO2). How many total atoms of oxygen are burned in the reaction (reactants), and how many total atoms of oxygen are produced in the reaction (products)?

34. Counting Atoms
We can also relate mass to the number of atoms and the mole.
Relative atomic masses and the mol can be used to develop a method of measuring the amount of a sample.
If we had a pile of C atoms that weighed 12 g and a pile of H atoms that weighed 1g
each pile would contain the same number of atoms or 6.02x1023 atoms

35. Counting Atoms
The gram atomic masses of any elements (since they are relative to C) must contain the same # of atoms
A pile of any atom that corresponds to its average atomic mass from the PT contains exactly 6.02x1023 atoms of that element.
And is equal to how much 1 mole of that sample would weigh in grams

36. 18.99g F atoms=1 mol F atoms=6.02x10 F atoms
Counting Atoms
What this allows us to do is to use the mass off of the periodic table to represent the mass of 1 mole (or 6.02x1023 atoms) of that element
1 mol of C atoms weighs g
1 mol of H atoms weighs g
1 mol of W atoms weigh g
18.99g F atoms=1 mol F atoms=6.02x10 F atoms 23

37. Counting Atoms
This new version of mass from the periodic table is called the gram molar mass, or molar mass.
Molar Mass = mass of 1 mole of atoms/molecules/or formula units in grams
abbreviated = MM
units = grams/mol

38. Counting Atoms
We get the mass of 1 mole of any element off of the Periodic Table
So how do we figure out the mass of a mole of a compound rather than just an element?
To answer that question you must have the formula of the compound.

39. Counting Atoms
What is the mass of 1 mole of Sulfur Trioxide (SO3) A.K.A. Molar Mass?
The formula of a compound tells you how many atoms of each element combine to make the representative particle of that compound.
Model
Formula SO 3
H O 2
2 Hydrogen atoms
per molecule
3 oxygen atoms
per molecule

40. Counting Atoms
You can calculate the mass of a mol-ecule of SO3 by adding the MM of each of the atoms in the molecule
The mass of 1 mole of S is 32.1g.
The mass of 3 atoms of O is 3 times the MM of a single O atom, or (3)(15.99g) = 47.97g
So MM of 1 molecule of SO is 32.1g g = 80.1 g/mol

41. 6.02x1023 molecules of the compound – it would weigh 180 grams
Counting Atoms
What is the MM of glucose (C6H12O6)?
(6C’S)(12.011g/mol) =
72.066g/mol
(12H’S)(1.008 g/mol) =
12.096g/mol
(6O’S)(15.99 g/mol) =
95.94g/mol
180.1g/mol
1 mole of C6H12O6 or
6.02x1023 molecules of the compound – it would weigh 180 grams

42. Practice Time:
Determine the mass of 1 mole (Molar mass) of each of the following: Zn Gd
Zn(C2H3O2)2
(NH4)3PO4
C12H22O11

43. How many grams are in 9.45 mol of Dinitrogen Trioxide (N2O3)
Counting Atoms
Using the mole in calculations:
How many grams are in 9.45 mol of Dinitrogen Trioxide (N2O3)
The only connection to N2O3 and its mass is how much 1 mol of the compound weighs. But, using it as a comparison we can calculate the mass of 9.45 mol.

44. Counting Atoms Step 1: Determine the mass of 1 mol of N2O3
(2N’s)(14.01g) =
28.02g
(3O’s)(15.99g) =
47.97g
75.99g
If 1 mole of N2O3 weighs 75.99g than how much will 9.45 mols of N2O3 weigh?

45. Counting Atoms 75.99 g N2O3 9.45 mol N2O3 1 mol N2O3 =718 g N2O3
Step 2: Convert the given moles to grams using the Molar mass equality.
75.99 g N2O3
9.45 mol N2O3
1 mol N2O3
=718 g N2O3

46. Counting Atoms Find the number of moles of
92.2g of Iron (III) Oxide (Fe2O3)
The only connection to the mass of Fe2O and how that corresponds to mols is how that 1 mol of the compound weighs a certain # of grams. Using that we can figure out how many moles weigh 92.2 g

47. Counting Atoms (2Fe’s)(55.85g) = 111.7g (3O’s)(15.99g) = 47.97g
Step 1: Determine the mass of 1 mol of Fe2O3
(2Fe’s)(55.85g) =
111.7g
(3O’s)(15.99g) =
47.97g
159.67g
If 1 mole of Fe2O3 weighs g than how many mols will weigh 92.2 g of Fe2O3?

48. Counting Atoms 1 mol Fe2O3 92.2 g Fe2O3 159.67 g Fe2O3 =.577mol Fe2O3
Step 2: Convert the given mass to mols using the Molar mass equality
1 mol Fe2O3
92.2 g Fe2O3
g Fe2O3
=.577mol Fe2O3

49. Practice Time:
Vitamin C (C6H8O6), cannot be stored by the body and therefore, must be present in the diet. If an orange contains 70 mg of Vit C how many oranges are necessary to consume 2.5 mols of Vit C?

50. Counting Atoms
Another generality about the mole is that with matter in the gas phase we can develop another equality.
Under the same conditions, equal volumes of gases contain equal numbers of particles.
1 mol of a gas will occupy the same volume as 1 mol of any other gas under the same conditions.

51. Molar Volume of a Gas
22.4 L
22.4 L
1 mole of any gas under the specific conditions of 0°C and 1 atm (STP) will occupy a volume of 22.4 L.
6.02x1023
molecules of O2
6.02x1023
molecules of CO2

52. What volume, in Liters, does 0.60 mol of SO2 gas occupy at STP?
Counting Atoms
What volume, in Liters, does 0.60 mol of SO2 gas occupy at STP?
The only connection to the volume of any gas and how that corresponds to mols is that 1 mol of a gas occupies 22.4 Liters of space. Using that we can figure out the volume of 0.60 mols of a gas at STP.

53. Counting Atoms 22.4 L SO2 0.60 moles SO2 1 mole SO2 = 13 L SO2
Step 1: Use the equality 1 mol = 22.4 L to do the conversion
22.4 L SO2
0.60 moles SO2
1 mole SO2
= 13 L SO2

54. Practice Time:
The Snoopy balloon featured at the Macy’s Thanksgiving Day Parade has a volume of ft3. How many moles of Helium are required to fill it STP?

56. Counting Atoms
If you have a 35.67g piece of Chro-mium metal on your car, how many atoms of Chromium do you have?
You are given mass and asked for number of particles
Let’s get some strategy

58. Counting Atoms = 4.130x1023 atoms of Cr 1 mole Cr 6.02x1023 atoms
1st convert given mass into moles
To do this we use the molar mass (MM) of Cr which on the PT is g/mol
Then we need to convert from mols to atoms using Avogadro’s number.
1 mole Cr
6.02x1023 atoms
35.67g Cr
51.996g Cr
1 mole Cr
= 4.130x1023 atoms of Cr

59. Calculate the Missing Info…
Formula
Molar Mass
Moles
Mass
Particles
Gas STP
CH4
1.5 mol
H2SO4
79.0 g CO
9.03x1023 molecules
Cu(NO3)2
93.8 g