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