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MASS CONCEPTS

(i) Atomic Mass: The atomic mass of an element is a number which indicates how many times an atom of natural isotopic composition of an element is heavier as compared with 1/12 of mass of a atom of carbon-12.

(ii) Gram atomic mass: The atomic mass of an element expressed in grams is called Gram atomic mass e.g., Atomic mass of oxygen = 16 a.m.u.

Gram atomic mass of oxygen = 16 g

Note: (1 a.m.u. = 1.6 x 10^-24 g)

(iii) Molecular mass: The molecular mass of a substance (element or compound) is the number of times, the molecule of substance is heavier than 1/12th of the mass of an atom of corbon-12 isotope.

(iv) Gram molecular mass: The molecular mass of a substance expressed in grams is called its Gram molecular mass.

1 gram molecule of H₂SO₄ = 98.0 g

(v) Atomic Mass Unit: The quantity 1/12th mass of and atom of carbon.12 is known as atomic mass unit.



A mole is defined as number of atoms in 12g of carbo-12, which is experimentally found to be 6.023 x 10₂₃.

Illustration 1: In naturally occurring neon, isotopes and their relative abundance are as follows:

Isotope Fractional Abundance

²⁰Ne 0.9051

²¹Ne 0.0027

²²Ne 0.0922

Average atomic mass of Ne is

(A) 20. 179

(B) 21.179

(C) 22. 179

(D) 20

Solution: (A) Average atomic mass of Ne = 20 x 0.9051 + 21 x 0.0027 + 22 x 0.0922 = 20.179

Aliter: From the fraction abundance of various isotopes, clearly shows that the average atomic mass should be near to 20 (²⁰Ne). Hence, (A) is correct

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