Chemistry Revision Notes Of Concept of Atomic, Molecular and Equivalent Masses for NDA

Atomic Mass

Carbon-12 is the carbon isotope that has six protons and six neutrons. Setting the atomic mass of carbon-12 at 12 amu provides the standard for measuring the atomic mass of the other elements. Therefore,

`text(Atomic mass) = text(Mass of an atom of the element)/(1/12th text( mass of one carbon atom) (C-12))`

It is expressed in atomic mass unit (amu or u). Atomic mass unit (u) is defined as `1/12` th mass of carbon atom C-12.

Average Atomic Mass

Most naturally occurring elements (including carbon) have more than one isotope. When we measure the mass of an element, we must generally settle for the average mass of the naturally occurring mixture of isotopes. If an element has three isotopes with atomic masses `m_1, m_2` and `m_3` and these occur in the ratio of `x, y` and `z` respectively, then its average atomic mass

`= ( m_1 xx x+ m_2 xx y + m_3 xx z)/(x+y+z)`

e.g. Boron occurs in nature in the form of two isotopes `B^(10) , B^(11)` with atomic mass 10 and 11 in the ratio of `1 : 4` respectively.
Therefore, average atomic mass of boron

` = (10xx1+11xx4)/(4+1) = (10+44)/5 = 54/5 = 10.8`

Gram Atomic Mass

It is the atomic mass of an element expressed in grams. It is also called gram atom. e.g. the atomic mass of oxygen is `16 u`, therefore gram atomic mass of oxygen is `16 g`.

Molecular Mass

Molecular formula of a compound is normally used for determining the molecular mass of that compound. The molecular mass is the sum of atomic masses of all the atoms present in that molecule. It is expressed in atomic mass unit (u).

e.g. Molecular mass of `H_2SO_4 = 2 xx` atomic mass of hydrogen `+` atomic mass of sulphur `+ 4 xx` atomic mass of oxygen `= 2xx 1 + 32+ 4xx 16 = 98`

Note : For ionic compounds like `NaCl` and `MgO` that do not contain discrete molecular units, the term formula mass is used in place of molecular mass.

Gram Molecular Mass

It is the mass of a substance expressed in grams. lt is also called as gram molecule.