Quantitative Composition of Compounds

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Quantitative Composition of Compounds
Preparation for College Chemistry Luis
Avila Columbia University Department of Chemistry
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Depending upon Bonding type
Compounds
Ionic (Coulombic forces)
Molecular (Covalent bonds)
Molecules
Cations
Anions
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Claude Berthollet
Proportions by mass of elements in a compound
VARY OVER A CERTAIN RANGE
Joseph Proust
Proportions by mass of elements in a compound ARE
FIXED. VARIATIONS ARE DUE TO IMPURITIES.
Careful experimentation lead Proust to
demonstrate
THE LAW OF DEFINITE PROPORTIONS (CONSTANT
COMPOSITION)
The proportions by mass of the elements in a
compound ARE FIXED, and do not depend on its mode
of preparation.
2HCl(g)
H2(g) Cl2(g)
2HCl(g) Na2SO4(aq)
H2SO4(l) 2NaCl(s)
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Certain SOLIDS are exceptions of the Law of
Constant Composition NON STOICHIOMETRIC
COMPOUNDS (BERTHOLLIDES)
Wüstite, an iron oxide whose simplest formula is
FeO, with 77.73Fe.
Its composition truly ranges from Fe0.95O
(76.8 Fe) to Fe0.85O (74.8 Fe) depending of
the method of preparation.
All gaseous compounds OBEY THE LAW OF DEFINITE
PROPORTIONS.
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The composition of a compound is shown by its
CHEMICAL FORMULA.
CHEMICAL ANALYSIS
Lets take the elements C and O
(1.000 g C and 1.333 g O)
A
C O2
(1.000 g C and 2.667 g O)
B
C O2
1.333 2.667 or 1 2
For a FIXED mass of C the ratio of O in A and B
is

If A is CO then B CO2
If A is CO2 then B is C2O4
We are unable to say which one is the right
formula, but we know the ratio C O is the
QUOTIENT OF INTEGERS.
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Molecules
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Composition
Held together by covalent bonds
Usually made up of nonmetal atoms
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Types of Formulas
CH3
Empirical
C2H6
Molecular
Structural
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Atomic and Formula Masses
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Masses of Individual Atoms

• The atomic masses of H, Cl, and Ni are
• H 1.008 amu
• Cl 35.45 amu
• Ni 58.69 amu
• Therefore 1.008g H, 35.45g Cl, and 58.69g Ni all
  have the same number of atoms NA
• NA Avogadros number 6.022 x 1023

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Meaning of Atomic Masses

• Give relative masses of atoms based on C12
  scale
• The Most common isotope of carbon is assigned
  an atomic
• mass of 12 amu.
• The amu is defined as 1/12 of the mass of one
  neutral carbon
• atom

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Masses of Individual Atoms

• Mass of H atom
• 1 H atom x 1.674 x 1024g
• Number of atoms in one gram of nickel
• 1.00g Ni x 1.026 x 1022 atoms

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Formula Mass
The formula for water is H2O. What is its molar
mass?
2H 2(1.008?g/mol)? ?2.016 g/mol 1O 1(16.00
g/mol) 16.00 g/mol
18.02 g/mol molar mass of water
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The Mole
Meaning
Molar Mass
Mole - Mass Conversions
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Meaning
1 mol 6.022 x 1023 items
Cl2 HCl H Cl
6.022 x 1023 molecules 6.022 x 1023 molecules 6.022 x 1023 atoms 6.022 x 1023 atoms
70.90 g Cl2 36.46g HCl 1.008g H 35.45g Cl
1 mol Cl2 1 mol HCl 1 at-gr H 1 at-gr Cl
1 molar mass Cl2 1 molar mass HCl 1 molar mass H 1 molar mass Cl

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Molar Mass

• Generalizing from the previous examples, the
  molar mass, M, is numerically equal to the
  formula mass

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Mole-Mass Conversions
Calculate mass in grams of 13.2 mol CaCl2
Calculate number of moles in 16.4g C6H12O6
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Calculating Composition
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Mass from Formula
Percent composition of potassium dichromate,
K2Cr2O7?
molar mass K2Cr2O7 (78.20 104.00
112.00)g/mol 294.20g/mol
Note that percents must add to 100
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Composition from Experimental Data
Aluminum chloride is formed by reacting 13.43 g
aluminum with 53.18 g chlorine. What is the
composition of the compound?

• Calculate mass of compound formed

• Divide mass of each element by total mass
  of compound
• and multiply by 100.

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Empirical Formula from Composition
Empirical formula of compound containing 26.6 K,
35.4 Cr, 38.0 O
work with 100g sample26.6 g K, 35.4 g Cr, 38.0
g O
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Empirical Formula from Composition
Note that 2.38 / 0.680 3.50 7 / 2
Empirical formula K2Cr2O7
Potassium Dichromate?
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Empirical Formula from Analytical Data
A sample of acetic acid (C, H, O atoms) weighing
1.000 g burns to give 1.446 g CO2 and 0.6001 g
H2O. Empirical formula?
Solution
find mass of C in sample (from CO2)
find mass of H in sample (from H2O)
find mass of O by difference
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Empirical Formula from Analytical Data
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Simplest Formula from Analytical Data
Empirical formula is CH2O
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Molecular Formula from Empirical Formula
Must know molar mass
Calculate empirical and molecular formulas of a
compound that contains 80C, 20H, and has a
molar mass of 30.00 g/mol.
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Molecular Formula from Empirical Formula
Divide each value by smaller number of moles
Empirical Formula CH3
Molecular Formula (CH3)2 C2 H 6