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Chem ch 2
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1
2Chemical Formulas
and Composition
Stoichiometry
2
Chapter Goals
1. Chemical Formulas
2. Ions and Ionic Compounds
3. Names and Formulas of Some Ionic
Compounds
4. Atomic Weights
5. The Mole
3
Chapter Goals
6. Formula Weights, Molecular Weights, and Moles
7. Percent Composition and Formulas of Compounds
8. Derivation of Formulas from Elemental Composition
9. Determination of Molecular Formulas
10.Some Other Interpretations of Chemical Formulas
11.Purity of Samples
4
Chemical Formulas
• Chemical formula shows the chemical composition of the substance.– ratio of the elements present in the molecule or
compound
• He, Au, Na – monatomic elements
• O2, H2, Cl2 – diatomic elements
• O3, P4, S8 - more complex elements
• H2O, C12H22O11 – compounds
5
Chemical Formulas
Compound 1 Molecule Contains
HCl 1 H atom & 1 Cl atom
H2O 2 H atoms & 1 O atom
NH3 1 N atom & 3 H atoms
C3H8 3 C atoms & 8 H atoms
6
Ions and Ionic Compounds
• Ions are atoms or groups of atoms that possess an electric charge.
• Two basic types of ions:– Positive ions or cations
• one or more electrons less than neutral
• Na+, Ca2+, Al3+
• NH4+ - polyatomic cation
– Negative ions or anions• one or more electrons more than neutral
• F-, O2-, N3-
• SO42-, PO4
3- - polyatomic anions
7
Ions and Ionic Compounds
• Sodium chloride
– table salt is an ionic compound
8
Names and Formulas of
Some Ionic Compounds• Table 2-1 gives names of several
molecular compounds.
– You need to know the names of these compounds
• Some examples are:
– H2SO4 - sulfuric acid
– FeBr2 - iron(II) bromide
– C2H5OH - ethanol
9
Names and Formulas of
Some Ionic Compounds
• Table 2-2 displays the formulas, charges,
and names of some common ions
– You must know the names, formulas, and
charges of the common ions in table 2-3.
• Some examples are:
– Anions - Cl1-, OH1-, SO42-, PO4
3-
– Cations - Na1+, NH41+, Ca2+, Al3+
10
Names and Formulas of
Some Ionic Compounds• Formulas of ionic compounds are determined by
the charges of the ions.– Charge on the cations must equal the charge on the
anions.
– The compound must be neutral.
• NaCl sodium chloride (Na1+ & Cl1-)
• KOH potassium hydroxide(K1+ & OH1-)
• CaSO4 calcium sulfate (Ca2+ & SO42-)
• Al(OH)3 aluminum hydroxide (Al3+ & 3 OH1-)
11
Names and Formulas of
Some Ionic Compounds
You do it!
• What is the formula of nitric acid?
– HNO3
• What is the formula of sulfur trioxide?
– SO3
• What is the name of FeBr3?
– iron(III) bromide
12
Names and Formulas of
Some Ionic Compounds
You do it!
• What is the name of K2SO3?
– potassium sulfite
• What is charge on sulfite ion?
– SO32- is sulfite ion
• What is the formula of ammonium sulfide?
– (NH4)2S
13
Names and Formulas of
Some Ionic Compounds
You do it!
• What is the charge on ammonium ion?
– NH41+
• What is the formula of aluminum sulfate?
– Al2(SO4)3
• What are the charges on both ions?
– Al3+ and SO42-
14
Atomic Weights
• Weighted average of the
masses of the constituent
isotopes of an element.
– Tells us the atomic masses of
every known element.
– Lower number on periodic
table.
• How do we know what the
values of these numbers
are?
15
The Mole
• A number of atoms, ions, or molecules
that is large enough to see and handle.
• A mole = number of things
– Just like a dozen = 12 things
– One mole = 6.022 x 1023 things
• Avogadro’s number = 6.022 x 1023
– Symbol for Avogadro’s number is NA.
16
The Mole
Example 2-1: Calculate the mass of a single Mg
atom, in grams, to 3 significant figures.
17
The Mole
Example 2-2: Calculate the number of atoms in
one-millionth of a gram of Mg to 3 significant
figures.
18
The Mole
Example 2-3: How many atoms are contained in
1.67 moles of Mg?
19
The Mole
Example 2-4: How many moles of Mg atoms are
present in 73.4 g of Mg?
You do it!
20
Formula Weights, Molecular
Weights, and Moles• How do we calculate the formula weight of
a compound?
– sum the atomic weight of each atom
The formula weight of propane, C3H8, is:
3 C 3 12.01 amu 36.03 amu
8 H 8 1.01 amu 8.08 amu
Forula Weight 44.11 amu
21
Formula Weights, Molecular
Weights, and MolesThe formula weight of calcium nitrate, Ca(NO3)2,
is:
You do it!
22
Formula Weights, Molecular
Weights, and Moles• One Mole of Contains
– Cl2 or 70.90g 6.022 x 1023 Cl2 molecules
2(6.022 x 1023 ) Cl atoms
– C3H8
You do it!
23
Formula Weights, Molecular
Weights, and MolesExample 2-5: Calculate the number of C3H8
molecules in 74.6 g of propane.
24
Formula Weights, Molecular
Weights, and MolesExample 2-6: What is the mass of 10.0 billion
propane molecules?
You do it!
25
Formula Weights, Molecular
Weights, and MolesExample 2-7: How many (a) moles, (b) molecules, and (c) oxygen atoms are contained in 60.0 g of ozone, O3? The layer of ozone in the stratosphere is very beneficial to life on earth.
You do it!
26
Formula Weights, Molecular
Weights, and MolesExample 2-7a: How many moles are contained in 60.0 g of ozone, O3?
27
Formula Weights, Molecular
Weights, and MolesExample 2-7b: How many molecules are contained in 60.0 g of ozone, O3?
28
Formula Weights, Molecular
Weights, and MolesExample 2-7c: How many oxygen atoms are contained in 60.0 g of ozone, O3?
29
Formula Weights, Molecular
Weights, and MolesExample 2-8: Calculate the number of O atoms in
26.5 g of Li2CO3.
You do it!
30
Formula Weights, Molecular
Weights, and Moles• Occasionally, we will use millimoles.
– Symbol - mmol
– 1000 mmol = 1 mol
• For example: oxalic acid (COOH)2
– 1 mol = 90.04 g
– 1 mmol = 0.09004 g or 90.04 mg
31
Formula Weights, Molecular
Weights, and MolesExample 2-9: Calculate the number of mmol in
0.234 g of oxalic acid, (COOH)2.
You do it!
32
Percent Composition and
Formulas of Compounds• % composition = mass of an individual
element in a compound divided by the total
mass of the compound x 100%
Determine the percent composition of C in C3H8.
33
Percent Composition and
Formulas of CompoundsWhat is the percent composition of H in C3H8?
You do it!
34
Percent Composition and
Formulas of CompoundsExample 2-10: Calculate the percent composition
of Fe2(SO4)3 to 3 significant figures.
You do it!
35
Derivation of Formulas from
Elemental Composition• Empirical Formula - smallest whole-number ratio of
atoms present in a compound– CH2 is the empirical formula for alkenes
– No alkene exists that has 1 C and 2 H’s
• Molecular Formula - actual numbers of atoms of each element present in a molecule of the compound– Ethene – C2H4
– Pentene – C5H10
• We determine the empirical and molecular formulas of a compound from the percent composition of the compound.– percent composition is determined experimentally
36
Derivation of Formulas from
Elemental Composition• We determine the empirical and molecular formulas of a
compound from the percent composition of the compound.– percent composition is determined experimentally
37
Derivation of Formulas from
Elemental CompositionExample 2-11: A compound contains 24.74% K, 34.76% Mn, and 40.50% O by mass. What is its empirical formula?
38
Derivation of Formulas from
Elemental CompositionExample 2-12: A sample of a compound contains
6.541g of Co and 2.368g of O. What is the
empirical formula for this compound?
You do it!
39
Determination of Molecular
FormulasExample 2-13: A compound is found to contain 85.63% C and
14.37% H by mass. In another experiment its molar mass is
found to be 56.1 g/mol. What is its molecular formula?
Law of Multiple Proportions
• It is possible for two elements, A and B, to
combine to form more than one
compound.
• The ratios of the masses of element B that
combine with a given mass of element A in
each compound can be expressed by
small whole numbers.
40
Law of Multiple Proportions
Example 2-14: Show that the compounds NO2
and N2O5 obey the law of multiple proportions.
41
Some Other Interpretations of
Chemical FormulasExample 2-15: What mass of phosphorous is
contained in 45.3 grams of (NH4)3PO4?
42
43
Some Other Interpretations of
Chemical FormulasExample 2-16: What mass of ammonium phosphate,
(NH4)3PO4, would contain 15.0 g of N?
44
Purity of Samples
• The percent purity of a sample of a
substance is always represented as
mass of pure substance% purity = 100%
mass of sample
- mass of sample includes impurities
45
Purity of Samples
Example 2-18: A bottle of sodium phosphate,
Na3PO4, is 98.3% pure Na3PO4. What are the
masses of Na3PO4 and impurities in 250.0 g of this
sample of Na3PO4?