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Class 9, Science, Chapter-3, Lecture-3, Empirical Formula (Notes)

EMPIRICAL FORMULA:

The formula which gives the simple whole number ratio of atoms of various elements present in one molecule of a compound is called its empirical formula.

MOLECULAR FORMULA:

The formula which gives the actual number of atoms of various elements present in one molecule of a compound is called its molecular formula.

FORMULA UNIT:

The empirical formula of an ionic compound or a covalent network solid compound which is used as an independent entity in chemical equations is termed as formula unit.
Examples:

  • Formula unit of ionic compounds
  • Sodium Chloride - ${\rm{NaCl}}$ ; Calcium Hydroxide - ${\rm{Ca}}{\left( {{\rm{OH}}} \right)_{\rm{2}}}$
  • Formula unit of covalent network solids
  • Silicon dioxide - ${\rm{Si}}{{\rm{O}}_{\rm{2}}}$ ; Graphite - ${\rm{C}}$

ATOMIC MASS:

The relative mass of an atom of an element as compared to ${{\rm{1}} \over {{\rm{12}}}}$th of the mass of an atom of carbon–12 ${{\rm{1}} \over {{\rm{12}}}}$ is termed as atomic mass.
 Relative atomic mass ${\rm{ = }}{{{\rm{mass~of~atom~of~an~element}}} \over {{{\rm{1}} \over {{\rm{12}}}}{\rm{ of~mass~of~}}{}^{{\rm{12}}}{\rm{C~atom}}}}$

  • Relative atomic mass is measured in unified mass (u) [Older unit – atomic mass unit (a.m.u)]

EMPIRICAL FORMULA MASS:

The relative mass of all the atoms present in an empirical formula of a substance as compared to ${{\rm{1}} \over {{\rm{12}}}}{\rm{th}}$ of the mass of an atom of carbon-12 $\left( {{}^{{\rm{12}}}{\rm{C}}} \right)$ is termed as empirical formula mass.

MOLECULAR MASS:

The relative mass of all the atoms present in a molecule of a substance as compared to ${{\rm{1}} \over {{\rm{12}}}}$th of the mass of an atom of carbon-12 $\left( {{}^{{\rm{12}}}{\rm{C}}} \right)$ is termed as molecular mass.

${\rm{Relative~molecular~mass~=~}}{{{\rm{mass~of~molecule~of`substance}}} \over {{{\rm{1}} \over {{\rm{12}}}}{\rm{~of~mass~of~}}{}^{{\rm{12}}}{\rm{C~atom}}}}$

FORMULA UNIT MASS:

The relative mass of all the atoms present in a formula unit of an ionic substance as compared to ${{\rm{1}} \over {{\rm{12}}}}$th of the mass of an atom of carbon-12 $\left( {{}^{{\rm{12}}}{\rm{C}}} \right)$ is termed as formula unit mass.

Relation Between Molecular Formula and Empirical Formula:

${{\rm{Molecular Formula   =   n  \times }}\left( {{\rm{Empirical Formula}}} \right)}$

Where, ${\rm{n   =   }}{{{\rm{Molecular Mass}}} \over {{\rm{Empirical Formula Mass}}}}$