1. Quantitative Composition of Compounds
Preparation for College Chemistry
Luis Avila
Columbia University
Department of Chemistry

2. Atomic and Formula Masses
Meaning of Atomic Masses
Atomic Mass from Isotopic Composition
Masses of Individual Atoms
Formula Mass

3. Meaning of Atomic Masses
Give relative masses of atoms based on C–12 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 = amu
Cl = amu
Ni = amu
Therefore 1.008g H, 35.45g Cl, and 58.69g Ni all have the same number of atoms: NA
NA = Avogadro’s number = x 1023

7. Masses of Individual Atoms
Mass of H atom:
1 H atom x = x 10–24g
Number of atoms in one gram of nickel:
1.00g Ni x = x 1022 atoms
1.008g H
6.022 x 1023 atoms
6.022 x 1023 atoms Ni
58.69g Ni

8. Formula Mass The formula for water is H2O. What is its molar mass?
2H = 2(1.008g/mol) = g/mol
1O = 1(16.00g/mol) = 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 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 H
1 molar mass Cl2
1 molar mass HCl
1 molar mass H
36.46g HCl

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
110.98g CaCl2
1 mol CaCl2
mass = 13.2 mol CaCl2 x = 1.47 x 103g
Calculate number of moles in 16.4g C6H12O6
1 mol C6H12O6
180.18g C6H12O6
moles = 16.4g C6H12O6 x = 9.10 x 10-2mol

13. Formulas % Composition from Formula
% Composition from Experimental Data
Empirical Formula from % Composition
Molecular Formula from Empirical Formula

14. Mass % from Formula
Percent composition of K2CrO7?
molar mass K2CrO7 = ( )g/mol = g/mol
78.20
242.20
%K = x 100 = 32.29%
52.00
242.20
%Cr = x 100 = 21.47%
112.00
242.20
%O = x 100 = 46.24%
Note that percents must add to 100

15. % Composition from Exp. Data
Aluminum chloride is formed by reacting g aluminum with 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 sample:26.6 g K, g Cr, g O
moles K = 26.6g x = mol K
1 mol
39.10g
moles Cr = 35.4g x = mol Cr
1 mol
52.00g

17. Empirical Formula from %
moles O = 38.0g x = 2.38 mol O
1 mol
16.00g
Note that 2.38 / = 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 g burns to give g CO2 and 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
2.02g H
18.02g H2O
mass H = g H2O x = g H
mass O = 1.00g – 0.394g – 0.067g = 0.539g O

20. Empirical Formula from Analytical Data
1 mol C
12.01g C
moles C = 0.394g C x = mol C
1 mol H
1.008g H
moles H = g H x = mol H
1 mol O
16.00g O
moles O = 0.533g O x = mol O
Empirical formula is CH2O

21. Molecular Formula from Empirical Formula
Must know molar mass
Calculate empirical and molecular formulas of a compound that contains
80%C, 20%H, and has a molar mass of g/mol.

22. Molecular Formula: (CH3 )2 = C2 H6
Divide each value by smaller number of moles
Empirical Formula: CH3
Molecular Formula: (CH3 )2 = C2 H6