Chapter 3 ? Stoichiometry this term refers to mass relationships and changes during a chemical reaction. Mass is conserved, but atoms in product are arranged differently than they are reactants. I) Mole ? A) molecules composed of atoms in definite proportion by mass and in specific ratio: butyric acid has 54.5% C, 36.4% O and 9.1 % H it also has the atoms C/H/O in the ratio 2/4/1 B) Mole is used as a measure of the amount of a substance. 1 mole is the number of 126C atoms in 12.00 g Notice it relates number of atoms to mass of those atoms in grams. It is always 6.02 x 1023 So 6.02 x 1023 atoms of 12C weigh 12.00 g Instead of looking at masses of isotopes, can use average atomic mass of atoms: Find Se on periodic table