Download Chapter 6 Chemical Composition

Chapter 6
Chemical Composition
Objectives
1. To understand the concept of average mass
2. To learn how counting can be done by weighing
3. To understand atomic mass and learn how it is
determined
4. To understand the mole concept and Avogadro’s
number
5. To learn to convert among moles, mass, and number of
atoms
Chapter 6
Chemical Composition
Some Questions………
• How many atoms are there in a bucket of water? How
could we determine that without having to count them all
individually?
• I know that one atom of carbon reacts with one molecule
of oxygen to make one molecule of carbon dioxide. If I
have a sample of carbon how can I predict how much
oxygen to react with it? Can I do it by counting out atoms?
C + O2 reacts to give CO2
• What do the atomic masses in the Periodic Table stand
for?
periodic table
Chapter 6
Chemical Composition
A. Counting by Weighing
• We can count objects by knowing how much each one
weighs and weighing the sample
• Objects do not need to have identical masses to be
counted by weighing.
– All we need to know is the average mass of the
objects.
• To count the atoms in a sample of a given element by
weighing we must know the mass of the sample and the
average mass for that element.
=
Chapter 6
Chemical Composition
A. Counting by Weighing
Averaging the Mass of Similar Objects
Example: How can we count out 1000 jelly beans?
1. Not all jelly beans have the same mass.
2. Suppose we weigh 10 jelly beans and find:
3. Now we can find the average mass of a bean.
4. Finally we can multiply to find the mass of 1000
beans. We can then weigh out 1000 beans.
Chapter 6
Chemical Composition
B. Atomic Masses: How Much Do Atoms Weigh?
• Atoms have very tiny masses so scientists
made a unit to avoid using very small
numbers.
• 1 atomic mass unit (amu) is 1/12 of the
mass of a 12C atom
• 1 amu = 1.66 10-24 g
How many amu’s does a 12C atom weigh?
How many grams does a 12C atom weigh?
Chapter 6
Chemical Composition
B. Atomic Masses: Counting Atoms by Weighing
• The average atomic mass for an element is the
weighted average of the masses of all the isotopes of an
element.
• The Periodic Table shows the atomic mass of each
element in amu’s. This is different to the mass number
periodic table
What is the average atomic mass in amu’s of an atom of:
Barium
Fluorine
Xenon
Nickel
(P210 Q5,6)
Chapter 6
Chemical Composition
Calculate the Average Atomic Mass of Naturally
Occurring Iron:
Isotope
Atomic Mass
amu
Natural
Abundance %
Abundance x
Atomic Mass
54Fe
53.9396127
5.845
3.153
56Fe
55.9349393
91.754
51.323
57Fe
56.9353958
2.119
1.206
58Fe
57.9332773
0.282
0.163
Sum of the final column = 55.845 amu
Chapter 6
Chemical Composition
Calculate the atomic masses of one of the following
elements. Use the isotope masses and % abundance
from the Dynamic Periodic Table – Cu, C, K
Chapter 6
Chemical Composition
The MOLE!!?!?!
- A spicy sauce flavored with chocolate, usually served with turkey or
chicken
- Any of various small insectivorous mammals, esp. of the family
Talpidae, living chiefly underground, and having velvety fur, very small
eyes, and strong forefeet.
- A spy who becomes part of and works from within the ranks of an
enemy governmental staff or intelligence agency.
- A large, powerful machine for boring through earth or rock, used in
the construction of tunnels
- A small, congenital spot or blemish on the human skin, usually of a
dark color, slightly elevated
- A very large number: 6.022 x 1023
-The atomic or molecular weight of a substance expressed in grams
Chapter 6
Chemical Composition
C. The Mole
Multiple Units
• One dozen eggs is twelve eggs
• One century is a hundred years
• One kilometer is 1000 meters
• One score is …………?
• One gross is………..?
• One Giga is………...?
Can you think of some other multiple units?
Chapter 6
Chemical Composition
C. The Mole
• One mole of anything contains 6.022 x 1023 units of that
substance.
– Avogadro’s number is 6.022 x 1023.
• A sample of an element with a mass equal to that
element’s average atomic mass (expressed in g)
contains one mole of atoms.
Chapter 6
Chemical Composition
C. The Mole
Chapter 6
Chemical Composition
One Mole of marbles would cover the USA to a
depth of approximately 60 km!!!
One marble has a diameter of 1cm and covers
approx. one sq cm
Area of USA is 10 million sq km
= 107 sq km = 1017 sq cm
Depth of marbles in a mole is 6 x 1023 / 1017
= 6 x 106 cm = 60 km
Chapter 6
Chemical Composition
If you had a ball of string that was 1 mole of
millimeters long would it reach around the Earth?
(The circumference of the Earth is 40,076 km)
Chapter 6
Chemical Composition
How many grams do the following weigh?
1.00 mol Se
2.00 mol Fe
72.5 mol Pb
0.102 mol Mg
0.521 mol Ni
1.23 x 10-3 mol Pt
Chapter 6
Chemical Composition
How many moles in the following samples?
26.2 g Gold
41.5 g Calcium
335 mg Barium
12.01 g Carbon
1.42 x 10-3 g Palladium
(P211 Q16)
Chapter 6
Chemical Composition
Relationships Between Moles, Mass and Atoms
Avogadro’s
Number
6.022x1023
Atoms
Atomic Mass
from Periodic
Table
Moles
Mass
Chapter 6
Chemical Composition
Summary
• One amu is one-twelfth of the mass of a 12C atom
• One amu is close to the mass of one proton or
one neutron.
• One amu is a very small mass – 1.66 x 10-24 g
• One mole is 6.022 x 1023 units of anything
• One mole (of atoms) of an element will have a
mass in grams equal to the mass in amu of one
atom of that element
• “One Mole” refers to 6.022 x 1023 atoms or to the
atomic mass expressed in grams of an element
Chapter 6
Chemical Composition
Some Questions………
• How many atoms are there in a bucket of water? How
could we determine that without having to count them all
individually?
• I know that one atom of carbon reacts with one molecule
of oxygen to make one molecule of carbon dioxide. How
many grams of oxygen react with 12 grams of carbon to
make carbon dioxide?
C + O2 reacts to give CO2
• How are the atomic masses in the Periodic Table
different to mass numbers?
Chapter 6
Chemical Composition
Objectives
1. To understand the definition of molar mass
2. To learn to convert between moles and mass
3. To learn to calculate the mass percent of an element in
a compound
Chapter 6
Chemical Composition
A. Molar Mass
• A compound is a collection of atoms bound together.
1 mole of
methane
1 mole of
carbon
and 4
moles of
hydrogen
• The molar mass of (“one mole of”) a compound is
obtained by summing the masses of the appropriate
number of moles of component atoms.
Chapter 6
Chemical Composition
A. Molar Mass
• For compounds containing ions the molar mass is
obtained by summing the masses of the component
ions. This is sometimes called the formula weight.
Chapter 6
Chemical Composition
What is the Molar Mass of the following Compounds?
Na3N
CS2
NH4Br
C2H5OH
H2SO3
H2SO4
MgSO4
Ni3(PO4)2
Chapter 6
Chemical Composition
A. Molar Mass
Calculations Using Molar Mass
• Moles of a compound (g) =
mass of the sample (g)
molar mass of the compound (
g
)
mol
• Mass of a sample (g) = moles of sample (mol) x
molar mass of compound (g/mol)
• Number of atoms in moles of a sample = number of moles x
number of atoms in formula x Avogadro’s number
Chapter 6
Chemical Composition
Relationships Between Moles, Mass and Atoms
Avogadro’s
Number
6.022x1023
Atoms
Atomic Mass
from Periodic
Table
Moles
Mass
Chapter 6
Chemical Composition
Relationships Between Moles, Mass, Molecules and
Atoms
Avogadro’s
Number
6.022x1023
Molecules
Molecular
Formula
Atoms
Molar Mass
using Periodic
Table
Moles
Mass
Chapter 6
Chemical Composition
B. Percent Composition of Compounds
• Percent composition consists of the mass percent of
each element in a compound:
Mass percent =
mass of a given element in 1 mol of compound
100%
mass of 1 mol of compound
What are the mass percentages of C, H and O in Ethanol?
Chapter 6
Chemical Composition
Calculate the Mass Percent of These Compounds
LiBr
CuOH
SrSO4
C6H12O6
C2H4O
C4H8O2
Chapter 6
Chemical Composition
Objectives
1. To understand the meaning of empirical formula
2. To learn to calculate empirical formulas
3. To learn to calculate the molecular formula of a
compound
Chapter 6
Chemical Composition
What is the Formula?
Mass Spectrometry
• If I analyze a compound by mass as 7.7% Hydrogen
and 92.3 % Carbon what is its formula?
• If I have 100g of the compound how many grams do
I have of each element?
• How many moles of each element do I have?
• What is the simple number ratio between the two
numbers of moles?
• What is the formula?
• Is this the only possible formula?
Chapter 6
Chemical Composition
A. Empirical Formulas
• The empirical formula of a compound is the simplest
whole number ratio of the atoms present in the
compound.
What is the empirical formula of the following
compounds?
• Sodium Peroxide
Na2O2
• Terephthalic acid
C8H6O4
• Phenobarbital
C12H12N2O3
• 1,4-dichloro-2-butene
C4H6Cl2
Chapter 6
Chemical Composition
B. Calculation of Empirical Formulas
Chapter 6
Chemical Composition
What is the Empirical Formula of a Compound
Containing…..
0.0806g Carbon, 0.01353 g Hydrogen, 0.1074 g Oxygen?
P212 Q36-39
Chapter 6
Chemical Composition
C. Calculation of Molecular Formulas
• The molecular formula is the exact formula of the
molecules present in a substance.
• The molecular formula is always an integer multiple of
the empirical formula.
Molecular formula = (empirical formula)n
where n is a whole number
Empirical Formula: CH
Molar Mass = 78
Molecular Formula?
Chapter 6
Chemical Composition
Formula Summary
For the sugar glucose