Methods of Determination of Atomic Mass

Methods of Determination of Atomic Mass

(i) Dulong and Pettit's method: According to Dulong and Pettit's law
Atomic mass Specific heat = 6.4 (approx.)
Atomic mass (approx.) =  6.4/Specific heat (in cals.)
This law is applicable to solid elements only except Be, B, C and Si because their specific heat is variable with temperature.
Atomic mass = Equivalent mass × Valency
Valency = (Approximate atomic mass/Equivalent mass)
(ii) Vapour density method: It is suitable for elements whose chlorides are volatile.
Valency of the element = Molecular mass of chloride/Equivalent mass of chloride   
= 2 × Vapour density of chloride/Equivalent mass of metal + 35.5

Atomic mass = Equivalent mass of metal Valency
(iii) Specific heat method: It is suitable only for gases. The two types of specific heats of gases are CP (at constant pressure) and  (at constant volume). Their ratio is known as g whose value is constant (1.66 for monoatomic, 1.40 for diatomic and 1.33 for triatomic gases).
          Atomic mass of a gaseous element = Molecular mass/Atomicity
(iv) Isomorphism method: It is based on law of isomorphism which states that compounds having identical crystal structure have similar constitution and chemical formulae.
Example: K2 SO4, K2 CrO4 and SeO4
(valency of S, Cr, Se = 6),
ZnSo4. 7H2O, MgSO4 7H2O, FeSO4 7H2O

(valency of Zn, Mg, Fe = 2).

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