CBSE Tutorials

Matter-Its Nature and Behaviour: Mole Concept

INTRODUCTION

Atomic Mass

· The average relative mass of an atom of an element as compared to the mass of an atom of carbon (C¹²) taken as 12. In other words atomic mass is a number which expresses as to how many times an atom of the element is heavier than 1/12^th of the mass of carbon atom. Therefore,

· Atomic mass =

Average Atomic Mass

· It has been found that majority of the elements occur as several isotopes. Isotopes are the different atoms of the same element possessing different atomic masses but same atomic number. The atomic mass of each isotope is determined separately and then combined according to the ratio of their proportions of occurrence. This is known as average atomic mass. E.g., Boron occurs in nature in the form of two isotopes B¹⁰, B¹¹ with atomic mass 10 and 11 in the ratio of 1 : 4 respectively. Therefore Average atomic mass of carbon

=

=

Gram Atomic Mass

· The atomic mass of an element expressed in grams is the gram atomic mass or it is also called gram atom e.g. the atomic mass of oxygen is 16 amu, therefore gram atomic mass of oxygen is 16g.

Molecular Mass

· Molecular mass of a compound is defined as the weight of a molecule of the compound relative to a carbon atom, the atomic weight of which is supposed to be equal to 12.

Molecular mass of H₂SO₄ =

2 atomic mass of hydrogen + atomic mass

of sulphur + 4 atomic mass of oxygen

= 2 1 + 32 + 4 16 = 98

Gram Molecular Mass

· Molecular mass of a substance expressed in grams is called gram molecular mass. It is also known as gram molecule. e.g., molecular mass of N₂ is 28 and its gram molecular mass is 28 gm.

Determination of the molecular weight

(i) By using Graham’s law of diffusion

r₁ and r₂ are the rates of diffusion of gases having molecular weights M₁ and M₂ respectively.

(ii) Victor Meyer’s method

V.D. = (Approx).

m grams of substance displaces x mL of air at NTP.

Equivalent Mass

• An equivalent mass of a substance is defined as the amount of it which combines with 1 mole of hydrogen atoms or replaces the same number of hydrogen atoms in a chemical reaction. The equivalent mass of a substance expressed in grams is called gram equivalent. e.g., in the compounds like H₂O, NH₃, CH₄, one mole of hydrogen combine with mole of O, mole of N and mole of C respectively Hence

Equivalent mass of O =

Equivalent mass of N =

Equivalent mass of C =

Relation between Atomic weight, Equivalent weight and Valency

Equivalent weight =

E =

e.g., equivalent mass of H₂SO₄ is = 49

For Bases

E =

For Ca(OH)₂, E =

For Salts

E =

cations or anions

For Oxides

E =

· In a redox reaction equivalent mass depend upon the number of electrons taking part in the reaction

E =

Where n factor is total change in oxidation number of element in molecule

Determination of Equivalent weight

(i) Hydrogen displacement method :

Eq. wt. of the metal

=

(ii) Oxide formation method :

Eq. wt. of element =

where w₁ = weight of element

w₂ = weight of chloride

(iii) Chloride formation method :

Eq. weight of the element =

where m₁ = weight of element

m₂ = weight of chloride

(iv) Metal displacement method : A more electropositive metal ‘B’ displaces less electropositive metal ‘A; from its salt solution.

=

Mole

· A mole is the amount of substance that contains as many particles as are present in 12 g of carbon – 12 isotope.

· ‘Mole may also be defined as “the amount of a substance in grams which contains 6.023 x 10²³ units of that substance.

· 6.023 x 10²³ particles of a substance weight equal to the molecular weigh of that substance in grams, therefore, 1 mole is the gram molecular weight of a substance :

Number of mole =

=

=

Gram Molar Volume

· The volume occupied by 1 mole of a substance is called gram molar volume of the substance. The gram molar volume of perfect gas is 22.4 liters at STP or NTP. N.T.P. Normal temperature and pressure S.T.P. Standard temperature and pressure e.g., volume of 1 mole of H₂ at STP = 22.4 L

STP = 22.4 L

Volume of mole of H₂,

= = 5.6 L

Vapour Density

· Vapour density of a gas is the ratio of the mass of a certain volume of the gas to the mass of equal volume of hydrogen, measured under same conditions of temperature and pressure.

Vapour density =

Molecular weight = 2 vapour density

Atomic weight = 2

Knowledge Update

– Equivalent weight of CO₂, SO₂ and SO₃ is equal to .

– Equivalent weight of acidic salt =

– Mole wt. = 2 vapour density

Mole =

Atomic wt. specific heat = 6.4 (approx)