Download Chapter 2 Stoichiometry–describes the quantitative relationships

CHEM 101 LECTURE NOTES
Monday, September 8, 2003
Fall 2003
Dr. Joy Heising
S 540-550
Chapter 2
Stoichiometry–
describes the quantitative
among elements
relationships
1. in compounds (composition stoichiometry)
2. in chemical changes (reaction stoichiometry)
Dalton’
s Atomic Theory –
summarized experimental
observations and interpretations in the nature of atoms:
1. an element is composed of extremely small,
indivisible particles called atoms
2. Atoms cannot be created, destroyed, or transformed
into atoms of another element
3. compounds are formed when atoms of different
elements combine with each other in small, wholenumber ratios.
4. the relative numbers and kinds of atoms are
constant in a given compound.
1
CHEM 101 LECTURE NOTES
Monday, September 8, 2003
Fall 2003
Dr. Joy Heising
S 540-550
Chemical formula –
the chemical composition of a
substance.
Some examples of chemical formulas for elements:
Monoatomic elements –
sodium (Na), copper (Cu), barium (Ba)
Diatomic elements –
N2, O2, F2, Cl2, Br2, I2, H2
More complex molecules –
S8, P4
For compounds, the formula indicates
1. elements present
2. the ratio in which the atoms of the elements occur
Some examples:
HCl –
1 H atom, 1 Cl atom
1 chemical formula
H2O -2 H atoms, 1 O atom
1 chemical formula
NH3 -1 N atoms, 3 H atoms
1 chemical formula
C3 H8 –
3 C atoms, 8 H atoms
1 chemical formula
2
CHEM 101 LECTURE NOTES
Monday, September 8, 2003
Fall 2003
Dr. Joy Heising
S 540-550
molecular compounds: (Table 2.2)
H2O2
hydrogen peroxide
NH3
ammonia
CH3CH2OH
methanol
CO2
carbon dioxide
Methane
CH4
benzene
C6H6
Ion –
an atom or group of atoms that carries an electrical
charge
Anion –
negatively charged ions (Cl-, O2-)
Cation –
positively charged ions (Na+, Ba2+)
Ionic compounds –
extended array of ions in which the total
positive and negative charges are equal (Fig 2.7)
Formula unit –the simplest whole number ratio of ions in
the compound (NaCl)
Is PO43- a molecule? No.
polyatomic anion.
Compound names and formulas (table 2-3)
NaCl
Sulfuric acid
Molecule
sodium chloride
H2SO4
?
NH4NO3
Na2SO4
ion
3
ammonium nitrate
sodium sulfate
CHEM 101 LECTURE NOTES
Monday, September 8, 2003
Fall 2003
Dr. Joy Heising
S 540-550
How much does a single atom weigh?
Different elements weigh different amounts –
related
to what makes them unique.
What units do we use to define the weight of an atom?
amu –
units of atomic weight. (atomic mass unit)
1 amu –
1/12 the mass of 1 “
carbon-12”
atom.
H
Mg
1 amu
24.3 amu
N
Ag
14 amu
107.8 amu
When scientists do experiments with substances (elements,
molecules, ionic compounds), they usually
- Use the chemical formula to plan (ratio of atoms, NaCl)
- Measure the amount needed in grams
How can we relate grams to the chemical formula?
Mole –
6.022 x 1023 atoms, molecules or formula units
Avogadro’
s number
**one mole of atoms of an element has a mass in grams
numerically equal to the atomic weight of the element**
1 H atom
1 mol H
1 amu
1.01 g
1 C atom
1 mol C
12 amu
12.01 g
4
CHEM 101 LECTURE NOTES
Monday, September 8, 2003
Fall 2003
Dr. Joy Heising
S 540-550
Why do we use such a weird number?
Because it gives us an ‘
easy’
relationship between
amus and grams.
How many atoms are contained in 1.67 moles of Mg?
How many grams does 1.67 moles of Mg weigh?
= 1.00 x 1024 atoms
= 40.6 grams
The ratio of atoms in the simplest formula for a compound
is the same as the ratio of moles of atoms of the elements in
a sample of the compound
1 H2O molecule
2 H atoms
1 O atom
1 mol H2O
2 mol H
1 mol O
How many molecules are there in one mole of propane?
6.022 x 1023 molecules
In one mole of chlorine (Cl2), what is the number of
a) Cl2 molecules? 6.022 x 1023 molecules
b) Cl atoms? 2(6.022 x 1023 molecules)
5
CHEM 101 LECTURE NOTES
Monday, September 8, 2003
Fall 2003
Dr. Joy Heising
S 540-550
In one mole of Calcium nitrate, what is the number of
a) formula units? 6.022 x 1023
b) Ca2+ cations? 6.022 x 1023
c) Nitrate anions? 2(6.022 x 1023)
How much does H2O weigh?
Formula weights –
the sum of the atomic weights of the
atoms in the formula.
Propane
C3 H8
3(12.01) + 8(1.00) = 44.03
Chlorine
Cl2
2(35.45) = 70.90
Calcium nitrate
Ca(NO3)2
40.08+ 2[14.01 + 3(16.00)] = 164.10
formula weight –
all compounds, molecules and elements
atomic weight –
atoms
molecular weight –
molecules
Molar mass –the mass of one mole of a substance. (g/mol)
Numerically equivalent to formula weight
(amu/formula unit)
6
CHEM 101 LECTURE NOTES
Monday, September 8, 2003
Fall 2003
Dr. Joy Heising
S 540-550
Calculate the number of C3H8 molecules in 74.6 g propane.
7
CHEM 101 LECTURE NOTES
Monday, September 8, 2003
Fall 2003
Dr. Joy Heising
S 540-550
Percent composition –the % of each element in the
compound (by mass)
ex. calculate the percent composition of C and H in C3H8.
Molar mass of C3H8
=
C
H
3 (12.01 g/mol)
8 (1.008 g/mol)
% carbon:
=
36.03 (g/mol)
44.09 (g/mol)
x
% hydrogen
=
8.064 (g/mol)
44.09 (g/mol)
x
=
=
36.03 (g/mol)
8.064 (g/mol)
44.09 (g/mol)
100
=
81.71%
100
=
18.29%
empirical formula –the smallest, whole number ratio of
elements present
NaCl
H2 O
H2O2 - HO
Molecular formula –actual number of atoms present in a
molecule
8
CHEM 101 LECTURE NOTES
Monday, September 8, 2003
Fall 2003
Dr. Joy Heising
S 540-550
If we have the % composition of a substance, we can
determine the empirical formula
Ex. 24.74% K
24.74 g K
34.76% Mn
34.76 g Mn
40.50% O
40.50 g O
100.00%
100 g substance
24.74 g K
x
1 mol K
39.10 g
=
0.6327 mol K
34.76 g Mn
x
1 mol Mn
54.94 g
=
0.6327 mol Mn
40.50 g O
x
1 mol O
16.00 g
=
2.531 mol O
divide through by smallest # moles to get the ratio:
0.6327 mol K/0.6327
0.6327 mol Mn/0.6327
2.532 mol O/0.6327
=1
=1
=4
empirical formula is KMnO4
but we still don’
t know the molecular formula unless we
also have the molar mass of the substance
ex. in previous example, if molar mass = 158 g/mol
empirical formula = molecular formula
9
CHEM 101 LECTURE NOTES
Monday, September 8, 2003
Fall 2003
Dr. Joy Heising
S 540-550
if a compound has the empirical formula HO
and its molar mass = 34.0 g/mol
HO
= 1.0 + 16.0 = 17.0 g/mol
34/17
=2
2(HO)
= H2O2
molecular formula
Chapter 2:
Composition stoichiometry –
the relative ratios of different
elements within one particular compound or molecule
Chapter 3:
Reaction stoichiometry –
the relative ratios between
different substances as they react with each other
10