RF/GCSU Page 1 of 3 Ch_03_Zumdahl_7th_ed.wpd
CHEM 1211 - Principles of Chemistry I
Chapter 3 - Stoichiometry
3.1 Counting by Weighing
!if the average mass of objects that can be treated as being identical (such as jelly beans) is
known then the number of objects in a given mass can be calculated
3.2 Atomic Masses
!the current system of atomic masses came into effect in 1961 and is based on C as the
12
standard
"in this system C is assigned a mass of exactly 12 atomic mass units (amu) and the
12
masses of all other atoms are given relative to this standard
!the mass spectrometer is used to accurately determine masses of atoms
!note the atomic masses in the periodic table; average of isotopes found in nature
3.3 The Mole
!because samples of matter typically contain so many atoms, a unit of measure called the
mole has been established for use in counting atoms (and molecules, ions, etc.)
!the mole (abbreviated mol) is defined as the number of atoms in 12 grams of pure carbon 12
!the number, determined by mass spectrometry is 6.022 × 10 ; this number is also known as
23
Avogadro's number
"note that this is a large number; 602,200,000,000,000,000,000,000; 6,000,000,000
people on earth; a trillion dollars is $1,000,000,000,000
"see Table 3.1
"the mass of 1 mole of an element is equal to its atomic mass in grams; hydrogen, H, is
one in the periodic table
3.4 Molar Mass
!a chemical compound is a collection of atoms
4
"CH , methane, is made up of 1 carbon atom and 4 hydrogen atoms; what is the mass of
one mole of methane molecules?
- from the periodic table one mol of carbon = 12.01 grams; 1 mol of hydrogen atoms
= 1.008 atoms; therefore the molar mass of methane is 16.04 grams
2324
#examples; H O (18.01 g/mol), CaCO (100.09 g/mol), H SO (98.09 g/mol)
