The quantity of a substance that contains as many elementary entities, such as atoms, molecules ions or formula units as the number of atoms in exactly 12 g of C-12. In modern terms, gram-molecule and gram-atom are termed as a mole of molecules and a mole of atoms respectively e.g., 1 gram-molecule of oxygen and I gram-atom of oxygen are expressed as 1 mole of O2 and 1 mole of O respectively.

**Mole:** A mole is the amount of a substance that contains as many species , such as atoms, molecules ions or other particles as there are atoms in exactly 12 g of C-12.

1 Mole of atoms = Gram atomic mass or molar mass = 6.022 x 1023 molecules = 22.4 L at NTP (OoC, 1 atm) = 22.7 L at STP (0°C, 1 bar)

**Atomic Mass:** Atomic mass of an element can be defined as the number which indicates how many times the mass of one atom of the element is heavier in comparison to ¹/12th part of the mass of Carbon-12

Atomic mass unit (amu): The quantity ¹/12 × mass of an atom of C-12 is known as atomic mass unit

(Standard molar volume is the volume occupied by 1 mole of any gas at NTP which is equal to 22.4 litres.)

Number of moles of atoms/molecules/ions/electrons

__Percentage Composition__

__Empirical and Molecular formula__Steps involved in the calculation of empirical formula :

- Convert the mass percentage into grams: considering 100 g of the compound, the given mass percentages represent the masses of the elements in grams.
- Calculate the number of moles of each element.
- Calculate the simplest molar ratio: divide the moles obtained by the least value from amongst the values obtained for each element.
- Calculate the simplest whole number ratio: multiply all the simplest atomic ratios by a suitable integer.
- Write the empirical formula: write the symbols of each element along with their whole number ratios side by side.

Molecular formula = empirical formula x n

n = 1,2,3,.....etc.