AOne atomic mass unit (1 Avogram) :

The quantity of mass equal to 1/12 th of the mass of an atom of carbon – 12

1 a.m.u. = 1.66X10^-24g

B .Atomic mass :

The atomic mass of an element is the number which shows how many times one atom of that element is heavier than 1/12 part of the mass of Carbon – 12 atom.

Atomic weight of an element is expressed in fractional value. Because naturally occurring elements usually exist as a mixture of several isotopes.

Ex : Chlorine is an isotopic mixture of and in the percentage ratio of 77 : 23.

The average relative atomic weight of chlorine = $\frac{35\times 77+37\times 23}{100}=35.46$

C. Molecular mass :

The molecular mass of a substance is the number which shows howmany times one molecular of the substance is heavier than 1/12 part of the mass of carbon – 12 atom.

D.Gram atomic mass or Gram atom :

The mass of Avogadro’s number of atoms of it in grams is known as gram atomic mass.

Ex : The atomic mass of oxygen = 16 a.m.u

Gram atomic mass of oxygen = 16 g

E. Gram molecular mass or Gram mole :

The mass of Avogadro’s number of molecules of it in grams is known as gram molecular mass.

Ex : Molecular mass of carbondioxide = 44 a.m.u

Gram molecular mass of carbondioxide = 44g.

F. Avogadro’s hypothsis :

Equal volumes of all gases under the same conditions of temperature and pressure contain equal number of molecules.

Applications :

i. It is used in determining the atomicity of elements.

ii. It is used in determining the relation between vapour density and molecular weight.

iii. It is used in finding the molecular formulae of gases.

G . S.T.P. or N.T.P. conditions :

0°C or 273 K temperature and one atomosphere or 760 mm of Hg or 76 cm of Hg pressure are known as standard conditions of temperature and pressure.

H. Gram Molar Volume (G.M.V.) :

The volume occupied by one gram molecular weight of any gas or vapour at S.T.P. is known as gram molar volume. It value is 22.4 Its. Or 22400 ml.

I. Avagadro’s Number (N) :

The number of molecules present in one gram molecular weight of a substance (or)

The number of atoms present in one gram atomic weight of an element (or)

The number of molecules present in one gram molar volume of a gaseous substance.

Its value = 6.203 X10²³.

J.. Loschmidt Number :

The number of molecules present in 1c.c. volume of a gas or vapour at S.T.P. is called Loschmidt number.

Its value = 2.617X10¹⁹

K. Mole :

A mole is the amount of substance which contains the same number of particle as there are atoms in 12 g. of carbon – 12. (or)

1 mole = 1 gram molecular weight

= 1 gram mole = 6.023X10²³ molecules

= Atomicity X 60.23X10²³atoms

= Occupies 22.4 It. Or 22400 ml at S.T.P. (For gases only)

= 6.023X10²³ formula units (For ionic compounds only

= 6.023 X 10²³ X The no. of electrons present in a molecule.

Q.

Molecular Weight Ionic compound contain the ions but not the molecules. So Formula weight is proposed for the ionic compounds.

Ex : Formula weight of sodium chloride = 58.5 a.m.u

Gram formula weight of sodium chloride = 58.5 g.

R.

No. of gram moles = $\frac{\text{Weight}\text{of}\text{the}\text{substance}}{\text{Gram}\text{molecular}\text{weight}}$

S.

No. of gram atoms = $\frac{\text{Weight}\text{of}\text{the}\text{element}}{\text{Gram}\text{atomic weight}}$

T.

Weight of single atom of an element = $\frac{\text{Gram}\text{atomic weight}}{6.023\times {10}^{23}}$

 

U.

Weight of single molecule of a substance = $\frac{\text{Gram}\text{molecular weight}}{6.023\times {10}^{23}}$

 

V.

The number of molecules in the given weight of the substance

$\frac{\text{Weight}\text{of the substance}\times \text{6}\text{.023}\times {\text{10}}^{\text{23}}}{\text{Gram}\text{molecular}\text{weight}}$

 

W.

 

The number of molecules in given volume (its) of the gas at S.T.P.$=\frac{\text{volume of the gas in litres}\times \text{6}\text{.023}\times {\text{10}}^{\text{23}}}{\text{22}\text{.4}}$