Lecture notes Chemistry 1C (CEM1CA1)

STOICHIOMETRY
Atomic Masses
The atomic masses are measured in atomic mass units (amu).
1 amu = 1.661 × 10-24g
Because atoms are too small to be expressed in gram
Example: 12C atom has an atomic mass of 12 amu.
Formula Weight: is the sum of all the atomic weights in a chemical formula. In
case of a molecule, it is known as molecular weight.
Atomic weight is the same as the atomic mass in the periodic table

The mole
 Normally atoms are too small.
 They group themselves for us to actually see them.
 The mol: is the amount of substance that contain as many
elementary particles as there are atoms in exactly 12 g of 12C.
 1 mole particle = 6.022 × 1023 particles (known as Avogadro’s number).
Particles can be molecules as well
 1 mole particle = 6.022 × 1023 molecules. This is known as Avogadro’s
number

Molar mass
Molar mass: is the mass in grams of 1 mole of a substance.
The units are gram per mole (g.mol-1 or g/mol).
Example 1: Calculate the molar mass of O atom
16
O atom weighs 16 amu or 16 u
Molar mass is 16 g or 16 g/mol
Example 2: Calculate the molar mass of H2SO4
Formula weight= 2× (AW of H) + (AW of S) + 4 × (AW of O)
= 2× 1 amu + 32.1 amu + 4 × 16.0 amu
= 98.1 amu