Lecture 6
Nomenclature
Chapter 9
24-September
Suggested HW
9.1, 9.5, 9.7, 9.11, 9.15, 9.17, 9.25, 9.29, 9.35, 9.39,
9.43, 9.51, 9.55, 9.57, 9.67, 9.69, 9.71, 9.73, 9.75,
9.79, 9.89, 9.95, 9.99, 9.101, 9.105, 9.109, 9.113,
9.115, 9.117, 9.119, 9.123, 9.127, 9.137, 9.139a
1
Law of Definite Proportions
Compounds
of Definite
Proportions
1. Combinations of two Law
or more
elements
connected through ionic or
covalent bonds
In a pure compound, the elements are ALWAYS present in the same definite
proportion
by massbut the atoms can be
2. Cannot be broken down by
physical means,
separated by chemical processes
3. Definite and constant elemental composition
2g blue
4g red
How much Blue if we have a
sample that is 8 g red?
Mass Percent = Mass of blue in compound
Total mass of compound
2
Law of Definite Proportions
3
Law of Definite Proportions
Mass of Cu (g) Mass of Se (g)
Mass of CuSe Mass of Cu left Mass of Se left Product Ratio
(g)
over (g)
over (g)
Cu:CuSe
Product Ratio
Se:CuSE
45.296
54.704
100.000
0.000
0.000
0.453
0.547
45.296
27.352
50.000
22.648
0.000
0.453
0.547
45.296
82.056
100.000
0.000
27.352
0.453
0.547
22.648
54.704
50.000
0.000
27.352
0.453
0.547
67.944
54.704
100.000
22.648
0.000
0.453
0.547
4
Formula Mass
Sum of atomic masses of all atoms present in one formula unit of a
substance, expressed in atomic mass units
cobalt
27
Co
Calculate the formula mass of:
58.933
CH4
NH4+
NH4(OH)
Riboflavin  C17H20N4O6
5
The mole and Avogadro's Number
The mass of 1 atom is WAY too small to be useful on a regular basis
Avogadro’s Number (NA)
1 mol = 6.022 x 1023 units
How many moles is 1 x 106
atoms of CH4
How many atoms are present in 0.05
moles of CH4?
6
The mole and Formula Mass
So why is this useful to chemists?
Argon
18
Formula mass = g mol-1
Ar
39.95
The mass of 1 Argon atom is 39.95 amu
How many moles of Ar are present in 25 g?
1 mole of Argon atoms = 39.95 g
Conversion factor!
How many Ar atoms are present in 25 g?
7
Molar Mass
Molar Mass = Formula Mass
For a given molecule, the molar mass is the sum of atoms
involved
CH4
NH4+
NH4(OH)
Riboflavin  C17H20N4O6
8
The Mole and Chemical Formulas
How many moles of Oxygen in 1 mole of:
CO
CO2
CO32-
Na2CO3
9
Summary of Mole Calculations
Number of
molecules
Avogadro’s
Number
Formula
Subscript
Number of
atoms
Moles of
the
molecule
Molar
Mass
Mass
Percent
Formula
Subscript
Avogadro’s
Number
Moles of
Atoms In a
Compound
Mass of the
molecule
Molar
Mass
Mass of the
atom
10
Some Sample Problems
You measure 10 g of sodium in a sample of Na2CO3.
How many moles of oxygen are present?
11
Sample Problems
What is the mass of Silicon (in grams) in:
4.444 x 1023 atoms of Si
1764 molecules of SiH4
12
Sample Purity
If a 63 g sample of K2SO4 is 72% pure:
How much of the sample is not K2SO4?
For every 100
g of a sample,
only 72 g is
K2SO4
How many moles of oxygen are present in the pure K2SO4?
13
Empirical Formulas
Chemical formulas that gives lowest whole number ratio of atoms
Molecular Formula
C2H6
Empirical Formula
C2H6O
C2H6O4
C10H30O16
C10H30O17
C1.67H2
C0.5H1O0.5
C0.25Cl1
C2.33H1.67O
14
Determining Empirical Formulas
Freon is
9.933% carbon
58.63% chlorine
31.44% fluorine
What is the Empirical Formula of Freon?
1. Assume 100g of freon
2. Determine the moles of each element
3. Divide all by the lowest number of moles
4. Multiply each number by an integer to get all to whole numbers
15
Determining Empirical Formulas
A compound is
26.85% potassium
35.35% chromium
38.06% oxygen
What is the Empirical Formula
16
Determining Molecular Formulas
You determined the empirical formula of a compound to be C2HCl.
This compound has a molecular formula 181.44 amu. What is the molecular formula?
You determined the empirical of a compound to be SNCl2.
This compound has a molecular formula 350.94 amu. What is the molecular formula?
17
Empirical and Molecular Formulas
Whole
Number
Ratio
Moles
of each
Element
Molar Mass
Empirical Formula
Mass Percentages
or
Actual Masses
Molecular Formula
Combustion
Analysis
18
Combustion Reactions
Combustion reactions occur when compounds react with O2 to produce CO2 and H2O
(and sometimes another biproduct)
CH4 +
O2

CO2 +
H2 O
If 18.032 g of methane is combusted, how much CO2 is produced?
How much H2O is produced?
19
Combustion Reactions
If a combustion reaction produces 0.294 g of CO2 and 0.120 g of H2O, what is the
empirical formula of the hydrocarbon?
CxHy +
O2

CO2 +
H2O
20