Examples: CuSO4 contains Cu - 1 atom, S - 1 atom, O - 4 atoms
A chemical formula represents the composition of a molecule of the substance in terms of the symbol of the elements present in the molecule. It is also called molecular formula.Molecular mass
The molecular mass of a substance is the relative mass of its molecule as compared with the mass of a12C atom taken as 12-units. It indicates the number of times, one molecule of the substance is
Table 2: Molecular Formulae of important Acids
| Acid | Molecular Formula |
|---|---|
| Hydrochloric acid | HCl |
| Nitric acid | HNO3 |
| Sulphuric acid | H2SO4 |
| Phosphoric acid | H3PO4 |
| Acetic acid | CH3COOH |
| Carbonic acid | H2CO3 |
| Sulphurous acid | H2SO3 |
| Nitrous acid | HNO2 |
Table-3: Molecular formulae of important bases
| Base | Molecular Formula |
|---|---|
| Ammonium hydroxide | NH4OH |
| Sodium hydroxide | NaOH |
| Potassium hydroxide | KOH |
| Calcium hydroxide | Ca(OH)2 |
| Magnesium hydroxide | Mg(OH)2 |
| Aluminium hydroxide | Al(OH)3 |
| Zinc hydroxide | Zn(OH)2 |
| Iron (II) hydroxide | Fe(OH)2 |
| Iron(III) hydroxide | Fe(OH)3 |
| Copper(II) hydroxide | Cu(OH)2 |
Molecular formulae of important salts
Table- 4: (A) Molecular formulae of Oxides
| Oxide | Molecular Formula |
|---|---|
| Sodium Oxide | Na2O |
| Potassium Oxide | K2O |
| Calcium Oxide | CaO |
| Magnesium Oxide | MgO |
| Aluminium Oxide | AlO |
| Zinc Oxide | ZnO |
| Iron(II) Oxide | FeO |
| Iron(III) Oxide | Fe2O3 |
| Lead(II)Oxide | PbO |
| Lead(IV)Oxide | PbO2 |
| Copper Oxide | CuO |
| Mercury Oxide | HgO |
| Carbonate or Hydrogen carbonate | Molecular Formula |
| Sodium Carbonate | Na2CO3 |
| Sodium hydrogen carbonate | NaHCO3 |
| Potassium carbonate | K2CO3 |
| Potassium hydrogen carbonate | KHCO3 |
| Calcium carbonate | CaCO3 |
| Calcium hydrogen carbonate | Ca(HCO3)2 |
| Magnesium carbonate | MgCO3 |
| Magnesium hydrogen carbonate | Mg(HCO3)2 |
| Aluminum carbonate | Al2(CO3)2 |
| Zinc carbonate | ZnCO3 |
| Iron(II) carbonate | FeCO3 |
| Lead(II) carbonate | PbCO3 |
| Tin(II) carbonate | SnCO3 |
| Copper(II) carbonate | CuCO3 |
Calculation of molecular mass
Molecular mass is equal to sum of the atomic masses of all atoms present in one molecule of the substance.Example:
H2OMass of H atom = 1
Mass of 2H atoms = 2Mass of O atom = 16
Molecular mass = 2 + 16 = 18gmsGram molecular mass
Molecular mass expressed in grams is numerically equal to gram molecular mass of the substance.Molecular mass of O2 = 32
Gram molecular mass of O2 = 32gmsSignificance of molecular formula
- It represents name of the substance
- It represents one molecule of substance
- It gives the names of all the elements present in one molecule of the substance. It also gives the number of atoms of each element present.
- It represents a definite mass and refers to the presence of 6.023 x 1023 molecule.
Relative atomic mass
It is very difficult to accurately weigh the number of atoms as they are extremely light and small and they cannot be seen. Indirect methods have been utilized to determine the absolute mass of an atom.
Relative atomic mass of elements
Mass of a hydrogen atom = 16.735 x 10-24gMass of a oxygen atom = 26.565 x 10-24g
We all know that atoms are too small and weigh too little to express their masses in grams or kilograms. Therefore it was agreed upon that the mass of a particular atom should be considered as a standard unit and the masses of other atoms to be related to the standard. The resulting masses are considered to be relative atomic masses as they are relative to the standard.Oxygen and hydrogen were initially chosen as the standard units. But later C12 atom [carbon isotope] was considered the standard for comparison with other atoms or molecules.
Relative atomic mass (RAM) of an element is the number of times one atom of an element is heavier than 1/12 the mass of an atom of carbon [C12].
