Chemistry IX | Matter-Its Nature and Behaviour: Mole Concept | Part 3

Matter-Its Nature and Behaviour: Mole Concept


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 1/12th of the mass of carbon atom. Therefore,

· Atomic mass = clip_image002

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

= clip_image004

= clip_image006

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 clip_image008 atomic mass of hydrogen + atomic mass

of sulphur + 4clip_image008[1] atomic mass of oxygen

= 2 clip_image008[2]1 + 32 + 4 clip_image008[3] 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. = clip_image016 (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 clip_image018 mole of O, clip_image020 mole of N and clip_image022 mole of C respectively Hence

Equivalent mass of O = clip_image024

Equivalent mass of N = clip_image026

Equivalent mass of C = clip_image028

Relation between Atomic weight, Equivalent weight and Valency

Equivalent weight = clip_image030

E = clip_image032

e.g., equivalent mass of H2SO4 is clip_image034 = 49

For Bases

E = clip_image036

For Ca(OH)2, E = clip_image038

For Salts

E = clip_image040

cations or anions

For Oxides

E = clip_image043

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

E = clip_image045

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

= clip_image047

(ii) Oxide formation method :

Eq. wt. of element = clip_image049

where w1 = weight of element

w2 = weight of chloride

(iii) Chloride formation method :

Eq. weight of the element = clip_image051

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.

clip_image053= clip_image055


· 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 = clip_image057

= clip_image059

= clip_image061

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. clip_image063 Normal temperature and pressure S.T.P. clip_image063[1] Standard temperature and pressure e.g., volume of 1 mole of H2 at STP = 22.4 L

STP = 22.4 L

Volume of clip_image022[1] mole of H2,

= clip_image067 = 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 = clip_image069

Molecular weight = 2 clip_image008[4] vapour density

Atomic weight = 2 clip_image008[5] clip_image073

Knowledge Update

– Equivalent weight of CO2, SO2 and SO3 is equal to clip_image075.

– Equivalent weight of acidic salt = clip_image077

– Mole wt. = 2 clip_image008[6] vapour density

Mole = clip_image080

Atomic wt. clip_image008[7] specific heat = 6.4 (approx)


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