Empirical and Molecular Formulae

Empirical and Molecular Formulae
1. Empirical (simplest ratio of atoms of each element that present in the compound) and molecular formulae (actual number of atoms of each element that are present in one molecule of the compound) indicate:

• the types of the elements
• the symbols of the elements and the ratio of atoms or
• moles of atoms of each element in a compound.

2. Molecular formula = (empirical formula)_n
n is a positive number

Compound	Molecular formula	n	Empirical formula
Carbon dioxide	CO2	1	(CO2) = CO2
Ethane	CH3	2	(CH3)2 = C2H6
Propene	CH2	3	(CH2)3 = C3H6
Glucose	CH2O	6	(CH2O)6 = C6H12O6
Quinine	C10H12NO	2	C20H24N2O2

3. Chemical formulae for covalent compounds.

Name	Chemical formula	Number of each element
Nitrogen gas	N2	2 nitrogen atoms
Oxygen gas	O2	2 oxygen atoms
Ammonia	NH3	1 nitrogen atom and 3 hydrogen atoms
Water	H2O	2 hydrogen atoms and 1 oxygen atom

4. Cations are positively-charged ions.

Charge	Cations	Formula
+1	Ammonium ion	NH4+
+1 *	Copper(I) ion	Cu+
+1	Hydrogen ion	H+
+1	Lithium ion	Li+
+1 *	Nickel(I) ion	Ni+
+1	Potassium ion	K+
+1	Silver ion	Ag+
+1	Sodium ion	Na+
+2	Barium ion	Ba2+
+2	Calcium ion	Ca2+
+2 *	Copper(II) ion	Cu2+
+2 *	Iron(II) ion	Fe2+
+2 *	Lead(II) ion	Pb2+
+2	Magnesium ion	Mg2+
+2 *	Manganese(II) ion	Mn2+
+2	Nickel(II) ion	Ni2+
+2 *	Tin(II) ion	Sn2+
+2	Zinc ion	Zn2+
+3	Aluminium ion	Al3+
+3 *	Chromium(III) ion	Cr3+
+3 *	Iron(III) ion	Fe3+
+4 *	Lead(IV) ion	Pb4+
+4 *	Tin(IV) ion	Sn4+

* refer to the Roman numerals
5. Anions are negatively-charged ions.

Charge	Anions	Formula
-1	Bromide ion	Br-
-1	Chloride ion	Cl-
-1	Chlorate(V) ion	ClO3-
-1	Ethanoate ion	CH3COO-
-1	Fluoride ion	F-
-1	Hydride ion	H-
-1	Hydroxide ion	OH-
-1	Iodide ion	I-
-1	Manganate(VII) ion	MnO4-
-1	Nitrate ion	NO3-
-1	Nitrite ion	NO2-
-2	Oxide ion	O2-
-2	Carbonate ion	CO32-
-2	Chromate(VI) ion	CrO42-
-2	Dichromate(VI) ion	Cr2O72-
-2	Sulphide ion	S2-
-2	Sulphate ion	SO42-
-2	Sulphite ion	SO32-
-2	Thiosulphate ion	S2O32-
-3	Nitride ion	N3-
-3	Phosphate ion	PO43-
-3	Phosphite ion	PO33-

6. Chemical formulae for ionic compounds

Name	Chemical formula	Number of cation	Number of anion
Zinc chloride	ZnCl2	1 Zn2+	2 Cl-
Copper(II) sulphate	CuSO4	2 Cu2+	2 SO42-
Aluminium sulphate	Al2(SO4)3	2 Al3+	3 SO42-

7. Meaning of prefixes

Prefix	Meaning
Mono-	1
Di-	2
Tri-	3
Tetra-	4
Penta-	5
Hexa-	6
Hepta-	7
Octa-	8
Nona-	9
Deca-	10

8. Naming of chemical (non-metal) compounds with Greek numerical prefixes.

Non-metal compound	Chemical formula
Carbon monoxide	CO
Carbon dioxide	CO2
Sulphur dioxide	SO2
Sulphur trioxide	SO3
Carbon tetrachloride (tetrachloromethane)	CCl4