Atomic mass

Atomic mass

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Atomic mass-

” Atomic mass of an element can be defined as the number which indicates how many times the mass of one atom of the element is heavier in comparison to 1/12^th part of the mass of one atom of carbon-12.”

Atomic mass of an element = mass  of one atom of the element  / (1/12^th ) part of the mass of one atom of carbon

Atomic mass unit-

It is represented by ‘amu’. It is equal to the quantity 1/12^th mass of an atom of C-12.

Actual mass of one atom of C-12 = 1.9924 x 10^-23 gm. = 1.9924 x 10^-26 Kg.

1 amu = 1.9924 x 10 ^-23/12 = 1.66 x 10 ^-24 gm. = 1.66 x 10 ^-27 Kg.

Atomic mass of an element = mass  of one atom of the element / 1 amu

Actual mass of an element = atomic mass of an element in amu x 1.66 x 10 ^-24 gm.

For ex-

Actual mass of hydrogen atom  = 1.008  x 1.66 x 10 ^-24 gm. = 1.6736 x 10^-24 gm.

Actual mass of oxygen atom  = 16.00  x 1.66 x 10 ^-24 gm. = 2.656 x 10^-23 gm.

Average Atomic weight (Mass)-

“The Average Atomic Masses of many elements are determined by multiplying the atomic mass of each isotope by its fractional abundance and adding these values and then dividing it by 100.”

Ex.- Naturally occurring carbon contains three isotopes – C¹² (98.892 % abundance), C¹³ (1.108 % abundance), C¹⁴ (2 x 10^-10 % abundance).The relative atomic masses of these isotopes are 12.000, 13.00335 and 14.00317 amu respectively.

Average atomic mass = % of I isotope x its atomic mass + % of II isotope x its atomic mass +% of III isotope x its atomic mass/100

= 12 x 98.892 + 13.00335 x 1.108 + 14.00317 x 2 x10^-10 /100

Average atomic mass= 12.011 amu

Molar volume –

Molar volume of a substance is volume occupied by 1 mole of that substance .

Molar volume of solid or liquid = molar mass / density

Molar volume of  ideal gas at 0⁰C or 273 K and 1 atmosphere pressure is 22.4 litre.

Gram atomic mass(weight) or gram atom –

“Gram atomic mass(weight) of an element is the mass of Avogadro number ( 6.023 x 10²³ ) of atoms of that elements in grams.”

Ex –

mass  of 1 atom of oxygen = 16 amu = 16 x 1.66 x 10^-24 gm

Mass  of 6.023 x 10²³ atoms of oxygen= 16 x 6.023 x 10²³ x 1.66 x 10^-24 = 16 gm.

Hence, Gram atomic weight of oxygen = 16 gm.

No. of gram atoms = mass of element in gm / atomic mass of element in gm.