Molecular Mass

The molecular mass of a substance is the sum of the atomic masses of all the atoms in a molecule of the substance.

Ex- Atomic mass of H₂O

= 2*Atomic mass of Hydrogen + 1*Atomic mass of Oxygen

= 2*1 + 1*16

= 2 + 16

= 18 u

Formula Unit Mass

• The formula unit mass of a substance is a sum of the atomic masses of all atoms in a formula unit of a compound.
• It is calculated in the same manner of molecules mass.

Ex- Calculate the formula unit mass of CaCl_2.

= Atomic mass of Ca + 2*Atomic mass of Chlorine

= 40 + 2*35.5

= 40 + 71

= 111u

Ions: An ion may be defined as an atom or group of atoms having positive or negative charge.

• Some positively charged ions : Na⁺, K⁺, Ca²⁺, Al³⁺
• Some negatively charged ions.

Cl^– ( Chlorine ion), S^2-( Sulphide ion)

We can classify ions in two types:

• Simple ions

1. Mg²⁺( Magnesium ion)
2. Na⁺ ( Sodium ion)
3. Cl^– ( Chloride ion)
4. Al³⁺ ( Aluminium ion)

• Compound ions

1. NH^4- (Ammonium ion)
2. CO₃^2-(Carbonate ion)
3. SO₄^2-(Sulphate ion)
4. OH^– ( Hydroxide ion)

Chemical formulae of ionic compounds(polyatomic)