1. Chapter 10
Chemical Composition

2. Counting by Weighing
Atoms are too small and there are too many to count individually.
Compare to M&M’s in a candy store.

3. Atomic Mass: Counting Atoms by Weighing
Because atoms are so small we can’t use the conventional units of grams and kilograms. Mass of a single carbon atom: 1.99 x g 1 atomic mass unit = 1.66 x 10-24g

4. AMU
Precisely,1/12 the mass of an atom of carbon-12. The carbon-12 (C-12) atom has six protons and six neutrons in its nucleus.
In imprecise terms, one AMU is the average of the proton rest mass and the neutron rest mass. This is approximately x 10 -27 kilogram (kg), or x 10 -24 gram (g). The mass of an atom in AMU is roughly equal to the sum of the number of protons and neutrons in the nucleus.

5. Atomic Mass: Counting Atoms by Weighing
Elements occur in nature as mixtures of isotopes
Carbon = 98.89% 12C
1.11% 13C
<0.01% 14C
Carbon’s average atomic mass = amu

6. The Mole
In the lab we will be measuring in grams not amu’s. You can consider them to be equivalent g Al = amu Al ? g Cu = amu Cu
63.66 g

7. The Mole
6.022 X 1023
To play the movies and simulations included, view the presentation in Slide Show Mode.

8. The Mole A counting unit
Similar to a dozen, except instead of 12, it’s 602 billion trillion 602,000,000,000,000,000,000,000
6.02 X 1023 (in scientific notation)
This number is named in honor of Amedeo _________ (1776 – 1856), who studied quantities of gases and discovered that no matter what the gas was, there were the same number of molecules present
Avagadro

9. Figure 8.1: All of these examples of pure elements contain the same number (a mole) of atoms: 6.022x1023 atoms.
Hmco Photo File

10. Just How Big is a Mole?
Enough soft drink cans to cover the surface of the earth to a depth of over 200 miles.
If you had Avogadro's number of unpopped popcorn kernels, and spread them across the United States of America, the country would be covered in popcorn to a depth of over 9 miles.
If we were able to count atoms at the rate of 10 million per second, it would take about 2 billion years to count the atoms in one mole.

11. Figure 8.2: One-mole samples of iron (nails), iodine crystals, liquid mercury, and powdered sulfur.
Hmco Photo File

12. Everybody Has Avogadro’s Number! But Where Did it Come From?
It was NOT just picked! It was MEASURED.
One of the better methods of measuring this number was the Millikan Oil Drop Experiment
Since then we have found even better ways of measuring using x-ray technology

13. Learning Check
Suppose we invented a new collection unit called a rapp. One rapp contains 8 objects.
1. How many paper clips in 1 rapp?
a) 1 b) 4 c) 8
2. How many oranges in 2.0 rapp?
a) 4 b) 8 c) 16
3. How many rapps contain 40 gummy bears?
a) 5 b) 10 c) 20

14. The Mole 1 dozen cookies = 12 cookies
1 mole of cookies = 6.02 X 1023 cookies
1 dozen cars = 12 cars
1 mole of cars = 6.02 X 1023 cars
1 dozen Al atoms = 12 Al atoms
1 mole of Al atoms = 6.02 X 1023 atoms
Note that the NUMBER is always the same, but the MASS is very different!
Mole is abbreviated mol (gee, that’s a lot quicker to write, huh?)

15. A Mole of Particles Contains 6.02 x 1023 particles
= x 1023 C atoms
= x 1023 H2O molecules
= x 1023 NaCl “molecules”
(technically, ionics are compounds not molecules so they are called formula units)
6.02 x 1023 Na+ ions and
6.02 x 1023 Cl– ions
1 mole C
1 mole H2O
1 mole NaCl

16. Avogadro’s Number as Conversion Factor
6.02 x 1023 particles
1 mole or
1 mole
Note that a particle could be an atom OR a molecule!

17. Learning Check 1. Number of atoms in 0.500 mole of Al a) 500 Al atoms
b) x 1023 Al atoms
c) x 1023 Al atoms
2.Number of moles of S in 1.8 x 1024 S atoms
a) mole S atoms
b) mole S atoms
c) x 1048 mole S atoms

18. Molar Mass The Mass of 1 mole (in grams)
Equal to the numerical value of the average atomic mass (get from periodic table)
1 mole of C atoms = g
1 mole of Mg atoms = 24.3 g
1 mole of Cu atoms = 63.5 g

19. A 1-mol sample of graphite (a form of carbon) weighs 12.01 g.
Hmco Photo File

20. Calculations with Molar Mass
Grams Moles
1 mol
mass (g)
mass (g)
1 mol

21. Converting Moles and Grams
Aluminum is often used for the structure of light-weight bicycle frames. How many grams of Al are in 3.00 moles of Al?
3.00 moles Al ? g Al

22. 1. Molar mass of Al 1 mole Al = 27.0 g Al
2. Conversion factors for Al
27.0g Al or mol Al
1 mol Al g Al
3. Setup 3.00 moles Al x g Al
1 mole Al
Answer = g Al

23. Learning Check! = 79.9 g/mole 1 mole of Br atoms 1 mole of Sn atoms
Find the molar mass (usually we round to the tenths place)
= 79.9 g/mole
1 mole of Br atoms
1 mole of Sn atoms
= g/mole

24. Other Names Related to Molar Mass
Molecular Mass/Molecular Weight: If you have a single molecule, mass is measured in amu’s instead of grams. But, the molecular mass/weight is the same numerical value as 1 mole of molecules. Only the units are different. (This is the beauty of Avogadro’s Number!)
Formula Mass/Formula Weight: Same goes for compounds. But again, the numerical value is the same. Only the units are different.
THE POINT: You may hear all of these terms which mean the SAME NUMBER… just different units

25. Molar Mass of Molecules and Compounds
Mass(g) of 1 mol = sum of the atomic masses
1 mole of CaCl2
1 mole Ca x 40.1 g/mol
+ 2 moles Cl x 35.5 g/mol
1 mole of N2O4
111.1 g/mol CaCl2
= 92.0 g/mol

26. Learning Check! 94.2 g/mol K2O 78.02 g/mol Al(OH)3
Molar Mass of K2O = ? Grams/mole
B. Molar Mass of antacid Al(OH)3 = ? Grams/mole
94.2 g/mol K2O
78.02 g/mol Al(OH)3

27. Learning Check
Prozac, C17H18F3NO, is a widely used antidepressant that inhibits the uptake of serotonin by the brain. Find its molar mass.

28. STOICHIOMETRY
- the study of the quantitative aspects of chemical reactions.

29. Learning Check!
The artificial sweetener aspartame (Nutra-Sweet) formula C14H18N2O5 is used to sweeten diet foods, coffee and soft drinks. How many moles of aspartame are present in 225 g of aspartame?

30. Atoms/Molecules and Grams
Since 6.02 X 1023 particles = 1 mole AND 1 mole = molar mass (grams)
You can convert atoms/molecules to moles and then moles to grams! (Two step process)
You can’t go directly from atoms to grams!!!! You MUST go thru MOLES.
That’s like asking 2 dozen cookies weigh how many ounces if 1 cookie weighs 4 oz? You have to convert to dozen first!

31. Everything must go through Moles!!!
Calculations
molar mass Avogadro’s number Grams Moles particles
Everything must go through Moles!!!

33. Atoms/Molecules and Grams
How many atoms of Cu are present in 35.4 g of Cu?
35.4 g Cu mol Cu X 1023 atoms Cu g Cu mol Cu
= 3.4 X 1023 atoms Cu

34. Learning Check!
How many atoms of K are present in 78.4 g of K?

35. Learning Check!
What is the mass (in grams) of 1.20 X 1024 molecules of glucose (C6H12O6)?

36. Learning Check! How many atoms of O are present in 78.1 g of oxygen?
78.1 g O2 1 mol O X 1023 molecules O2 2 atoms O g O mol O molecule O2

37. Calculating Formula Mass
Calculate the formula mass of magnesium carbonate, MgCO3.
24.31 g g + 3(16.00 g) =
84.32 g

38. Calculating Percentage Composition
Calculate the percentage composition of magnesium carbonate, MgCO3.
From previous slide:
24.31 g g + 3(16.00 g) = g
100.00

39. Percent Composition
What is the percent carbon in C5H8NO4 (the glutamic acid used to make MSG monosodium glutamate), a compound used to flavor foods and tenderize meats?
a) %C
b) %C
c) %C

40. Formulas molecular formula = (empirical formula)n [n = integer]
Empirical formula: the lowest whole number ratio of atoms in a compound.
Molecular formula: the true number of atoms of each element in the formula of a compound.
molecular formula = (empirical formula)n
[n = integer]
molecular formula = C6H6 = (CH)6
empirical formula = CH

41. Formulas (continued)
Formulas for ionic compounds are ALWAYS empirical (lowest whole number ratio).
Examples:
NaCl
MgCl2
Al2(SO4)3
K2CO3

42. Formulas (continued)
Formulas for molecular compounds MIGHT be empirical (lowest whole number ratio).
Molecular:
H2O
C6H12O6
C12H22O11
Empirical:
H2O
CH2O
C12H22O11

43. Empirical Formula Determination
Base calculation on 100 grams of compound.
Determine moles of each element in 100 grams of compound.
Divide each value of moles by the smallest of the values.
Multiply each number by an integer to obtain all whole numbers.

44. Empirical Formula Determination
Adipic acid contains 49.32% C, 43.84% O, and 6.85% H by mass. What is the empirical formula of adipic acid?

45. Empirical Formula Determination (part 2)
Divide each value of moles by the smallest of the values.
Carbon:
Hydrogen:
Oxygen:

46. Empirical Formula Determination (part 3)
Multiply each number by an integer to obtain all whole numbers.
Carbon: 1.50
Hydrogen: 2.50
Oxygen: 1.00
x 2
x 2
x 2 3 5 2
Empirical formula:
C3H5O2

47. Finding the Molecular Formula
The empirical formula for adipic acid is C3H5O2. The molecular mass of adipic acid is 146 g/mol. What is the molecular formula of adipic acid?
1. Find the formula mass of C3H5O2
3(12.01 g) + 5(1.01) + 2(16.00) = g

48. Finding the Molecular Formula
The empirical formula for adipic acid is C3H5O2. The molecular mass of adipic acid is 146 g/mol. What is the molecular formula of adipic acid?
2. Divide the molecular mass by the mass given by the emipirical formula.
3(12.01 g) + 5(1.01) + 2(16.00) = g

49. Finding the Molecular Formula
The empirical formula for adipic acid is C3H5O2. The molecular mass of adipic acid is 146 g/mol. What is the molecular formula of adipic acid?
3. Multiply the empirical formula by this number to get the molecular formula.
3(12.01 g) + 5(1.01) + 2(16.00) = g
(C3H5O2) x 2 =
C6H10O4

50. Chemical Formulas of Compounds (HONORS only)
Formulas give the relative numbers of atoms or moles of each element in a formula unit - always a whole number ratio (the law of definite proportions).
NO atoms of O for every 1 atom of N
1 mole of NO2 : 2 moles of O atoms to every 1 mole of N atoms
If we know or can determine the relative number of moles of each element in a compound, we can determine a formula for the compound.

51. Types of Formulas (HONORS only)
Empirical Formula
The formula of a compound that expresses the smallest whole number ratio of the atoms present.
Ionic formula are always empirical formula
Molecular Formula
The formula that states the actual number of each kind of atom found in one molecule of the compound.

52. To obtain an Empirical Formula (HONORS only)
1. Determine the mass in grams of each element present, if necessary.
2. Calculate the number of moles of each element.
3. Divide each by the smallest number of moles to obtain the simplest whole number ratio.
If whole numbers are not obtained* in step 3), multiply through by the smallest number that will give all whole numbers
* Be careful! Do not round off numbers prematurely

53. require mole ratios so convert grams to moles
A sample of a brown gas, a major air pollutant, is found to contain 2.34 g N and 5.34g O. Determine a formula for this substance.
require mole ratios so convert grams to moles
moles of N = 2.34g of N = moles of N
14.01 g/mole
moles of O = g = moles of O
16.00 g/mole
Formula:
(HONORS only)

54. Calculation of the Molecular Formula (HONORS only)
A compound has an empirical formula of NO2. The colourless liquid, used in rocket engines has a molar mass of 92.0 g/mole. What is the molecular formula of this substance?

55. Empirical Formula from % Composition (HONORS only)
A substance has the following composition by mass: % Na ; % B ; % H
What is the empirical formula of the substance?
Consider a sample size of 100 grams
This will contain grams of B and grams H
Determine the number of moles of each
Determine the simplest whole number ratio