ATOMIC,MOLECULAR AND EQUIVALENT MASSES/PHYSICAL AND CHEMICAL CHANGES ~ GENERAL STUDIES 4 U|FOR ALL COMPETITIVE EXAMS

BLEND OF ATOMS AND MOLECULES

ATOMIC MASS

Is the mass of atom and is defined as how many times an atom of the element is heavier than 1/12 of the mass of Carbon-12 atom.

MOLECULAR MASS

Is the mass of a molecule.Molecular mass is an additive property and is calculated by adding the atomic masses of total atoms present in a molecule e.g., NH3=14+(1)✕3=17.

EQUIVALENT WEIGHT

Is obtained by dividing molecular and or atomic mass by valency.

Equivalent weight =Molecular mass or atomic mass/Valency.

(i.e.,Equivalent weight is affected by change in Valency)

MOLE CONCEPT

The number of molecules present in 12g of carbon-12 is called one mole.
1 mole =6.022✕10²³=Avogadro's number (NA).
Number of moles=Mass (in g)/Atomic or molecular mass (g mol. inverse).
1 g-atom=1 mole atom =6.022✕10²³ atoms.
1 g-molecule =1 mole molecules=6.022✕10²³ molecules.

PHYSICAL AND CHEMICAL CHANGES

In physical changes, only the physical properties of matter like colour, hardness,density etc.,changes while the chemical properties and composition remain the same.

Crystallization,boiling,dissolution of salt and sugar,vaporization,burning of candle etc.,are examples of physical change.
In chemical change,the chemical composition as well as chemical properties of the matter change and a new substance is formed. Burning of any substance,photosynthesis,ripening of fruits etc.,are examples of chemical change.
Physical changes are reversible (i.e.,can be inverted to obtain the original substance ) while chemical changes are irreversible.