1. Chapter 3:
Stoichiometry
3.1 & 3.2 Atomic Masses
3.3 The Mole
3.4 Molar Mass
3.5 Percent Composition
3.6 Chemical Formulas

2. Atomic Mass

3. Atomic Mass
Atoms are so small, it is difficult to discuss how much they weigh in common units.
The decimal numbers on the table are atomic masses in amu.
Atomic masses are not decimals because they are based on averages of atoms & of isotopes.
The average atomic mass is from the mass of the isotopes and their relative abundance.

4. Atomic Mass Examples
There are two isotopes of carbon 12C with a mass of amu(98.892%), and 13C with a mass of amu (1.108%).
There are two isotopes of nitrogen, one with an atomic mass of amu and one with a mass of amu. What is the percent abundance of each?

5. Moles

6. Avogadro’s Number The mole is a number.
A very large number, but still, just a number.
6.022 ´ 1023 of anything is a mole
A large dozen.

7. 1 mole of 12C has 6.02 ´ 1023 atoms and a mass of 12 g
Avogadro’s Number
1 mole of 12C has 6.02 ´ 1023 atoms and a mass of 12 g

8. Molar Mass
By definition, these are the mass of 1 mol of a substance (i.e., g/mol).
The molar mass of an element is the mass number for the element that we find on the periodic table
Often called molecular weight or formula weight (in g/mol)

9. Using Moles
Moles provide a bridge from the molecular scale to the real-world scale.

10. Mole Relationships
One mole of atoms, ions, or molecules contains Avogadro’s number of those particles.
One mole of molecules or formula units contains Avogadro’s number times the number of atoms or ions of each element in the compound.

11. Percent Composition

12. mass of element in sample
Percent Composition
One can find the percentage of the mass of a compound that comes from each of the elements in the compound by using this equation:
% element = ´ 100
mass of element in sample
total mass of sample

13. Percent Composition So the percentage of carbon in ethane, C2H6, is…
(2)(12.0 amu)
(30.0 amu)
24.0 amu
30.0 amu =
x 100
= 80.0%

14. Empirical Formulas

15. Calculating Empirical Formulas
One can calculate the empirical formula from the percent composition.

16. Calculating Empirical Formulas
The compound para-aminobenzoic acid (you may have seen it listed as PABA on your bottle of sunscreen) is composed of carbon (61.31%), hydrogen (5.14%), nitrogen (10.21%), and oxygen (23.33%). Find the empirical formula of PABA.

17. Calculating Empirical Formulas
Assuming g of para-aminobenzoic acid,
C: g × = mol C
H: g × = 5.09 mol H
N: g × = mol N
O: g × = mol O
1 mol
12.01 g
14.01 g
1.01 g
16.00 g

18. Calculating Empirical Formulas
Calculate the mole ratio by dividing by the smallest number of moles:
C: =  7
H: =  7
N: = 1.000
O: =  2
5.105 mol
mol
5.09 mol
1.458 mol

19. Calculating Empirical Formulas
These are the subscripts for the empirical formula:
C7H7NO2

20. Combustion Analysis
Compounds containing C, H, and O are routinely analyzed through combustion in a chamber like this
C is determined from the mass of CO2 produced.
H is determined from the mass of H2O produced.
O is determined by difference after the C and H have been determined.

21. Elemental Analyses
Compounds containing other elements are analyzed using methods analogous to those used for C, H and O.

22. Molecular Formulas

23. Empirical To Molecular Formulas
Empirical is lowest ratio.
Molecular is actual molecule.
Need Molar mass.
Ratio of empirical to molar mass will tell you the molecular formula.
Must be a whole number because...

24. Example
A compound is made of only sulfur and oxygen. It is 50.0% S by mass. Its molar mass is 192 g/mol. What is its formula?

25. Chapter 3:
Stoichiometry
3.1 & 3.2 Atomic Masses
3.3 The Mole
3.4 Molar Mass
3.5 Percent Composition
3.6 Chemical Formulas