Laws of Chemical Combinations

The formation of compounds from elements is governed by five laws:

1) Law of conservation of mass

2) Law of definite proportions

3) Law of multiple proportions

4) Gaylussac’s law of volumes

5) Avagadro’s law

1) Law of conservation of mass:

• According to lavoisier, matter can neither be created nor be destroyed.
• Masses of reactants is equal to masses of products.

Example:  C      +       O₂      —–>     CO₂

12gm          32gm                   44gm

2) Law of definite proportions:

According to proust, a given compound always contains exactly the same proportion of elements by weight.

Example: In CO₂  molecule and H₂O molecule,we have the definite proportion of  oxygen i.e; 16gm of oxygen will be participating to form such compounds.

3) Law of multiple proportions:

According to Dalton,if two elements are combining to form more than one compound,the masses of one element combine with a fixed mass of other element in the ration of their whole numbers.

Example:  NO      —    16gm

N₂O₂  —    32gm

N₂O₃   —   48gm

N₂O₄   —   64gm

N₂O₅    —  80gm

4) Gaylussac’s law of volumes:

According to gaylussac, the reactants are combining with respect to its volume ratio to form products.

Example: 2H₂  +  O₂   ——–> 2H₂O

Here consider 1 mole of reactant is nothing but 100ml of  substance

That means 200ml of H₂  combines with 100ml of  O₂to form 200ml of H­₂O.

That means hydrogen and oxygen are going to combine in their fixed ratio of volumes as 2:1 ratio.

5) Avagadro’s law:

• According to avagadro,equal volumes of all gases at all temperatures and pressure should contain equal number of
• Here avagadro made a distinction amongs the atoms and molecules very clearly.

Example: 2H₂  +  O₂   ——–> 2H₂O

In this example, 2N molecules of hydrogen was present and

4N atoms of hydrogen was present.

1N molecules of oxygen was present and

2N atoms of oxygen was present.

DALTONS ATOMIC THEORY:

According to Dalton,

• Matter is composed of small indivisible particles called atoms.
• Atoms of different elements differ in mass in their fixed ratio and forms compound.
• Chemical reactions involve reorganization of atoms.

Demerit:

Could not explain the gaseous law of volumes.