Preparation for College Chemistry

1
Quantitative Composition of Compounds
Preparation for College Chemistry Luis
Avila Columbia University Department of Chemistry
2
Atomic and Formula Masses
3
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

4
Atomic Mass from Isotopic Composition
5
Atomic Mass from Isotopic Composition
20.00 (0.9092)
21.00 (0.0026)
22.00 (0.0882)
20.18 amu
A.M. Ne 20.18g/mol
6
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

7
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

8
Formula Mass
The formula for water is H2O. What is its molar
mass?
2H 2(1.008g/mol) 2.016 g/mol 1O
1(16.00g/mol) 16.00 g/mol
18.02 g/mol molar mass of water
9
The Mole
Meaning
Molar Mass
Mole - Mass Conversions
10
Meaning of Mole
1 mol 6.022 x 1023 items
Cl2
HCl
H
Cl
6.022 x 1023 molecules
6.022 x 1023 atoms
6.022 x 1023 atoms
6.022 x 1023 molecules
36.46g HCl
1.008g H
35.45g Cl
70.90 g Cl2
1 mol Cl2
1 mol HCl
1 at-gr H
1 at-gr H
36.46g HCl
1 molar mass H
1 molar mass HCl
1 molar mass Cl2
11
Molar Mass

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

12
Mole to Mass Conversion
Calculate mass in grams of 13.2 mol CaCl2
Calculate number of moles in 16.4g C6H12O6
13
Formulas
14
Mass from Formula
Percent composition of K2CrO7?
molar mass K2CrO7 (78.20 52.00 112.00)g/mol
242.20g/mol
Note that percents must add to 100
15
Composition from Exp. 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.

16
Empirical Formula from
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
17
Empirical Formula from
Note that 2.38 / 0.680 3.50 7 / 2
Empirical formula K2Cr2O7
Potassium Dichromate
18
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
19
Empirical Formula from Analytical Data
20
Empirical Formula from Analytical Data
Empirical formula is CH2O
21
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.
22
Divide each value by smaller number of moles
Empirical Formula CH3
Molecular Formula (CH3 )2 C2 H6